This is a genuine, non-trolling question. I'm curious.
I went to see a band a few days ago at a medium-sized venue (a few hundred capacity). The guitarist was using a Marshall full-stack. The sound coming out of the amp itself was almost as loud as the sound coming out of the PA, meaning that the sound guy had no way of controlling the mix. You could barely hear the vocals, and the sound changed as you walked around the venue (depending on whether the amp was pointing at you or not).
It got me thinking: under what circumstances is a huge amp actually useful in the 21st century?
In a massive arena, obviously no amp is loud enough and you need to run the sound through the PA. So you might as well have a small amp.
In a medium sized venue, having a big amp makes the sound worse as you're competing with the PA. So you're better off with a small amp.
In a tiny venue, you can't fit a massive Marshall stack in anyway, and if you're playing in pubs then you probably don't have a big enough van to drive one round in. And I think the previous point applies anyway - if the venue has a PA (which it must do if you have a singer - surely all venues that put live music on have PAs these days?) then it will be much easier to get a decent mix if your amp isn't fighting with the PA.
At home.. well, that's fairly obvious.
So.. what am I missing? Why doesn't everyone have a small amp and rely on the PA for their live amplification?