I believe the most prevalent use of YouTube is for entertainment purposes. I check daily for new videos from the Game Grumps, and every once in a while I watch one of Epicnamebro's many video game lore videos. Add to Northinlion's "Binding of Isaac" let's plays and several other videos I get the inkling to watch and you know the majority of my background noise while I'm playing my games. I used to use YouTube for listening to music, but the introduction of grooveshark.com and slacker radio, to at least some extent, eliminated that.
In addition to entertainment, YouTube is an incredibly useful tool for learning. Several websites make it possible for anyone to publish a set of instructions, but it is difficult to follow text only tutorials (along with the issue of reliable information). With YouTube, it is far easier to find the instructions you want in video form. Videos are much easier to follow, and you can see if the instructions actually work before trying it out yourself.
As far as uploading videos, I have in the past, but I deleted most of them. Lately, I've been collecting the things I need to do a let's play of my own. I have no idea how good it will turn out, but I might as well give it a shot. Most of the tools are at my disposal now, I just need a game to play.