In Czechoslovakia, there were nominally a few political parties other than the Communist Party of Czechoslovakia, but like you point out, they were basically puppets controlled by the communists and mostly just provided the illusion of political plurality, with all permitted political entities being grouped in the so called National Front. As to why would someone join these parties, fundamentally the answer would be the same as even outside the context of a communist regime – it varies. After all, you have people voting for parties with no hopes of achieving anything to this day.

The origins of the National Front can be traced back to the Second World War. In the spring of 1945, the communists and their sympathizers, a strong political force at the time with the backing of the Soviets, met with their 'pro-western' counterparts under president Edvard Beneš in Košice, Slovakia, to agree on something called the Košice Programme. What the non-communist side under Beneš had hoped to be a joint effort between the two sides aimed at maintaining Czechoslovakia or perhaps even returning it to its pre-war state turned out to be basically yet another step in the shift of power into the hands of the communists, with most of the agreements being prepared days in advance in Moscow.

One of the results of the Košice meeting was the establishment of a political coalition that would govern post-war Czechoslovakia – the National Front. Only the parties represented in it would be allowed to be a part of the government in a show of unity, a reasonable enough concept at the time, at least on paper. More importantly, despite the growing communist influence, it was not yet the monolithic block it would become after the events of 1948. People participated in politics in a broader spectrum, not confined by the communists.

This, of course, changed after the 1948 coup. The National Front became nothing more than a front. The Communist Party of Czechoslovakia seized power, directly absorbed some parties and elements of the Front – such as the Social Democrats – and pressured other parties, most notably the Christian Democrats, to undergo pro-communist reforms and purges, which they did, sometimes undergoing serious internal strife in the process. Most of the vocal anti-communist opponents in the leadership of these parties either emigrated or were hounded out soon after the coup, some even being imprisoned and executed.

That, however, still left tens of thousands of supporters of the non-communist parties, for example the Christian Democrats still retained a membership of some forty thousand well into the eighties. Now that in the great scheme of things is not much, and virtually all positions of power came to be dominated by the communists or their sympathizers. Furthermore, short of submitting a blank vote, there was no way of not voting for the (communist controlled) National Front candidates available, and even then the candidates from these puppet parties would not be represented in significant numbers.

Still, it would be these parties that would also occasionally serve as the breeding ground for whatever limited dissent could be mustered during the forty years of communist rule in Czechoslovakia. The Christian Democrats were fairly active in the hopeful but ultimately failed reform movements of the sixties, especially the younger members; some were vocal proponents of the Charter 77; and in the eighties they would draw the ire by organizing overt religious events and ramping up the calls for greater reform and liberalization. Indeed it would be the proponents of the latter that would form the core of the party after the Velvet Revolution in 1989.

So yes, of course it might have been advantageous to join the Communist Party proper, but, as alluded to in the first paragraph, advantage is not necessarily the decisive factor in elections or aligning with a political entity. The remaining membership of these parties would vary from their traditional supporters, to those with no quarrel with the idea of a socialist state but prefering the theoretical difference between communists proper and the parties in question, to genuine if quiet and quixotic proponents of autonomy and opposition – albeit few in number and with basically no possibility of achieving their goals in that framework and under the constant threat of clashing with the regime.

If you will allow some speculation, I feel that when talking about authoritarian regimes (and, actually, not only those) it is all too easy to slip into thinking of them in comfortable but ultimately misguided labels, painting everything with the same brush, portraying them as monolithic – they were all communists, they literally controlled everything, and so on and so forth – and that this sort of applies here.

Of course, it is true enough that when viewed from the outside, it is difficult to argue against the fact that it was the Communist Party that held the reins and called the shots and did in fact control the system. But ultimately that system is made up of people, individuals with their own agendas, thoughts, motivations – and political beliefs. Constrained and shaped by their historical context, but not non-existent.

In Poland just after the war some people that before the war were socialist believed this new country will be democratic and socialist.

So they stayed with pre-war socialist party - PPS, or the pre-war farmers' party - PSL. For example Stanisław Mikołajczak, important pre-war politician and member of "Polish government on exile in London" during WW2 - returned to Poland and was a ministry of farming in temporary government after WW2. He was also a leader of PSL. After elections falsified by communists he has been warned that he will have his immunity revoked and be accused of treason and sentenced to death - so he escaped to USA. That was obviously fortunate for communists - they got rid of him and could portrait him as a traitor and take over PSL.

TL; DR: It wasn't immediately clear to everybody that the new government will be totalitarian soviet puppet, also some of these parties had good reputation from before the war.