To the people that don't understand the whole Youtube cover thing:
I do it on a regular basis, and I also make tabs to the songs I play.
The video acts as a reference for the tab so they can see exactly what's being played, when, and how it should sound.
On another note; I do it only for Asian music, which tends to lack any sort of tablature, or even decent covers for that matter.
Some people do it for the same reason I do: To help other people play along to the songs.
Some people do it to try and gain exposure (Most people do, and it does help)
And most people do it because they enjoy it. They enjoy the feedback that they get, which can help them to improve technique etc.