A pretty big question being asked here (and a bit loaded) but I will try to answer it, at least to the best of my ability while on a smoke break.

Short answers
Why?: As you stated to prevent the rise of communist regimes (though more accurately: Soviet aligned/sympathetic governments)

Was it necessary?: This is essentially a counterfactual question that cannot actually be answered beyond speculation.

Longer answer: The cold war was very much a conflict of influence and spheres of influence, similar to the imperial days of old. It was beneficial for each power to have as many countries and people in their sphere as possible for economic, military, and diplomatic reasons. As such the two powers had many interests in foreign countries and their conduct. For example the Soviet Union had a great interest in a buffer zone between it and the western powers, hence the Warsaw Pact/establishment of communist countries in Eastern Europe, and crackdowns on any popular sentiment or resistance efforts which may undermine the political status quo of this buffer zone (IE the Hungarian revolt of '56, or East Germany in '53). While these actions were quite widely condemned and criticized in the west, the west themselves took very similar measures on multiple occasions.

For example: the Guatemalan coup d'etat of the 1954, which was sponsored by the US and overthrew a democratically elected president in favor of a right-wing dictatorship. The reasons for this were many but the primary root motive was President Arbenz land reform campaign. Basically Arbenz planned to nationalize and redistribute unused agricultural land being held by many private estates and most notably by the United Fruit Company (now known as Chiquita) to the farmers of Guatemala who were largely poor and landless. This was against US interests in two-ways: firstly it was against the interests of United Fruit Company and thus would negatively affect the finances of a major US corporate interest. Secondly a land redistribution campaign, while not inherently communist, was generally a more left-leaning policy/position. This meant that, in the eyes of the US government, Arbenz could potentially be sympathetic to the USSR/could realign itself to be in the sphere of the USSR. This would mean a country in the Soviet sphere just south of the US itself, which was a nightmare in the eyes of the US (hence their reaction to the Cuban revolution a few years later). Rather than risk this change in the diplomatic status quo, a right-wing dictatorship which was ardently anti-communist would be preferable since Arbenz was popular and democratically elected.

To put it simply: if the US could stop the rise of soviet influence/socialist popularity through more gentle means, then it would (IE funding the conservative parties in the Italian election of 1948, or funding US aligned countries with the Marshall plan) then it would. If more harsh means were needed to keep countries/people "in-line" as it were, then those would be used too (IE Guatemala, Chilean coup d'etat, funding anti-communist death squads in Latin America). This was largely a matter of pragmatism (carrot vs. stick) and is not unique to the US, the Soviets did so too only more openly in many instances. Much of the US interference and actions taken against the third world have only come to public consciousness after the end of the cold war, most Americans at the time would have little-to-no knowledge of the US role in the Guatemalan coup for example (assuming they knew that anything happened in Guatemala at all).

I don't really have time to go into more specific details at the moment, but let me know if you have more specific questions about this that I could elaborate on rather than just an overview of US practices during the Cold War.