small stage gear

Being new to this I need to ask what gear do small club/bar bands use? Just trying to get an understanding you know. I was going to buy one of those Mesa Boogie mini rectifier amps but then the guy said, and not that I didn't have any knowledge of this, but he said most groups run their sound out of PA system. Well okay and I have a powered mixer that I can use but what are these guys doing/ why all the powerful amps for sale? I mean if you are going thru a PA and I assume it's amplified why would you need a guitar or keyboard amp? Wouldn't you go-guitar, effects, mixer then PA amp? Or do they mic a guitar amp and then into a mixer? But wouldn't the amp sound mess up what's coming out of the PA speakers?