Using "folks" instead of "people" or similar is not mandated by any rule or policy.
It is merely a colloquially nice and neutral term with some familiarity to it that can be used to adress groups of people. In that vein, it is seen as "warmer" and more kind then "people", and has settled into common parlance with many people.
Additionally, hearing other people use certain mannerisms causes us all to gravitate towards these as well, especially when we respect the people using them. That is how this proliferates, in a very mild, non policy way. We're all prone to a bit of code switching whereever we talk, so when you get someplace and see a lot of people using that word, and if you have no particular preference, you gravitate towards using it.
Also, "folks" and "y'all" are an easy way to bypass grammar traps for us ESL people :)