I'm going to assume you mean the terror orchestrated by Stalin that took place following the assassination of Sergei Kirov on 1 December, 1934 which reached its zenith with the Red Army purges in 1938.

Were these purges a necessity to prevent a rightist (counterrevolutionary) coup? In a word, no. Following the civil wars, the execution of the imperial family, the end of foreign intervention in the USSR, and the recognition of her new government by some of the major powers the direct threat of a white coup was neutralized. The Bolsheviks possessed the largest and most battle hardened army in Eurasia by that time (it being the post war years), controlled Russia's cities, had overthrown the Kerensky Republican Government with almost laughable ease and had consolidated all real political power from the Soviet councils on the ground to the Politburo on high. The Bolsheviks were the rulers of the Soviet Union.

This leads us to a more interesting question: Were the purges necessary to consolidate the practical implementation of a specific interpretation of Marxism as espoused by Lenin within the Soviet Union as the dictatorship of the proletariat transitioned first to an industrialized, class conscious society, then to socialist utopia? Potentially yes.

Let me take a moment to point out the differences in Marx's theory versus the reality that faced the nascent Soviet Union in the early 1920s. Marx posited that only a fully industrialized society would be able to produce class conscious workers in sufficient numbers to successfully overthrow the bourgeoisie middle class - rather than attempt to join its ranks - which would then give the working class the ability to seize the means of production and liquidate or reeducate the aristocracy and capitalists. Marx envisioned this taking place in autocratic Central Europe since the working class of Western Europe had representative governments (which Marx detested as a mere pretense of sociopolitical power sharing the true purpose of which was to distract the workers with petty politics while maintaining the economic status quo), and that once the world saw the working classes of one nation seize the means of production that world revolution would follow more or less spontaneously. Once that had happened the work of creating a classless utopia of plenty would be simple work for the international class conscious proletariat.

After the Red victory in the Russian Civil War however, the Bolsheviks were faced with a failed attempt at exporting the revolution via the Red Army's botched invasion of Poland, and no serious revolutions taking place in Central Europe. So the great Socialist Revolution was confined to backward, agrarian, non industrialized Russia which lacked a class conscious proletariat and saw no short term prospect of world revolution.

Now the Bolshevik had to answer questions both about the Party's organization and what to do with their gains in the former Russian Empire, but they were flailing around in murky waters outside of Marxist thought. If there was no world revolution should we actively encourage one, or consolidate a workers utopia in one country? While waiting for the revolution do we develop Russia towards industrialism in order to foster class consciousness, or simply reeducate? How? What does a socialist economy look like when it must exist in one nation (this would prove to be a question that dogged all socialist revolutionaries world wide and one which was never seriously addressed by Marx, nor solved by his adherents). What is the party's role in Russia and the international workers movement? How does a revolutionary party deal with actual state governance? Do we encourage other national parties to foment revolution according to local circumstance, or follow the proven Bolshevik method? Are all parties equal, or are the Bolsheviks in command of the movement since they have an actual state at their disposal?

Marx was silent on these questions, and so the party split amongst the Rightists who generally viewed parliamentary debate within the party and party consensus to answer these questions while focusing on building Russia into a socialist state; while the Leftists saw the avante guarde theoreticians as the vanguards of the party who's leadership should be deferred to and who saw the world revolution as the only way forward. Lenin chose a centrist (that is, the center of the extreme left of the Bolsheviks) path: an all powerful party led by committed hardcore revolutionaries who made the decisions for both the party and the state. Once these decisions were made they became unquestionable party dogma which was combined with a ruthless rigid party discipline which mercilessly rooted out heterodoxy. This was thought necessary to see through the radical reforms to be implemented in the new Soviet State, which needed to be in an unassailable position from which it could export the revolution.

Now we come to Stalin (after much tangential explanation). Stalin was a party whip and enforcer, and someone who excelled at the humdrum of organization for both party and state, something the leftist intellectuals thought beneath them. For his years of support and proven loyalty during the civil war as a political commissar at the front, Stalin was given what was thought to be a minor role in the politburo: General Secretary. It became his job to organize the apparatus of government and staff it with proven party people. This placed a truly Machiavellian mind at the confluence of the State and the all powerful Party, one which he used to staff with young radicals and his loyal commissar cronies turning the government and its multiple overlapping security organs into his personal political tools.

Now Stalins motivations become murky. He certainly played first the Rightists in the Party against Trotsky and His Leftists before cynically turning against his erstwhile allies on the Right and eliminating them in turn. He certainly held grudges against many, and Trotsky in particular, and was the major - and eventually sole - benefactor of the elimination of the dissenting cliques inside the Party. However, both of those cliques were contrary to Leninism as interpreted by Stalin and his loyalist, and it does genuinely appear that Lenin was the only man Stalin respected and actually admired. Even Molotov years later would say that Lenin was harder than Stalin due to his unwillingness to compromise (not exactly something that comes to mind when we think of Stalin, but that's what he said), and on the surface it would appear that Stalin simply took Lenin's vision to its logical conclusion. This would include the eradication of all couter-Leninist thought as decided by the Central Committee and Politburo: the Vanguard of the Party.

In a way, a civil war within the Party was sort of an inevitability. One interpretation or another was going to eradicate the other, in typical revolutionary bloody fashion those espousing what was found to be "counterrevolutionary" would pay with their lives - a tangible example that the revolution only moved forward, even if it should eclipse its most ardent supporters who had become moderate while not actually changing any of their political or revolutionary views (Robespierre would be all too familiar with this notion). So, by combining the latent theoretical inconsistencies between Marxism and the realpolitik world the Bolsheviks inhabited there would be violent differences on the course to follow with dissent and factions in a minority party which sat tenuously atop a shaky, unstable state surrounded by enemies both real and potential while feeling themselves entrusted with the very destiny of humanity going forward: an intolerable state of affairs. Party discipline must reign unchallenged to survive, and see Marx's vision for a more egalitarian future for humanity. Whichever faction ended up taking control of the heights of the Party would do so with blood on their hands, and lists of those that would then be expelled or purged for the greater good. It just so happens that the most cynical and least ideological of all was placed in the position to best profit from the Party Lenin left behind, and generations of ideologically driven men would pay the ultimate price for it. The purges cemented his position after an intra-party war over ideology.

I know this has gone way beyond the scope of what was asked, but I hope it has given some idea of why the Bolshevik Party went the way it went in the end. Thanks for reading.

edit: Recommended reading:

Stalin: the Court of the Red Tsar - Simone Sebag Montefiore

Stalin: A biography - Robert Service

For the Soul of Mankind: The United States, The Soviet Union and The Cold War - Melvyn P. Leffler

Lenin, Stalin and Hitler - Robert Gellately

The Global Revolution: a History - Silvio Pons

European Communism 1848-1991 (European History in Perspective) - Ronald Kowalski