Hi and welcome to ASC.
As for answers to your questions...
The history lesson is pretty much covered but as to what people prefer etc that's really up to the individual airsofter. Some like wearing a full kit for even skirmishes and some guys go with the bare minimum to carry their primary plus a few mags.
Milsims generally have you carrying a lot more for the simple fact that you're not going to have any scheduled down time or lunches etc. You should be self sufficient enough to last the 24 or 48 or however long the Milsim scenario is.
Didn't really give any answers did I?