The term was arguably coined by RMS and his full statement was:

> “Free software” means software that respects users' freedom and community. Roughly, it means that the users have the freedom to run, copy, distribute, study, change and improve the software. Thus, “free software” is a matter of liberty, not price. To understand the concept, you should think of “free” as in “free speech,” not as in “free beer.”

https://www.gnu.org/philosophy/free-sw.html

Sometimes beer happens to be free, in which case it is referred to as "free beer". It's just an example.

I've always understood "free as in beer" as: if someone hands you a beer and says it's free, you know that you don't have to pay to consume the beer, but that doesn't mean that you also get the recipe, brewing instructions, factory plans, glass making instructions etc. The only thing that is free is the liquid itself, nothing else.

Lots of occasions, mostly celebrations or campaigns. It even has its own Wikipedia article:

https://de.m.wikipedia.org/wiki/Freibier

often when you are with friends