if u want to play enter sandman etc u dont need to now any music theory just pick up some tab and go for it. if on the other hand u want to be able to play a song in a similar style to enter sandman etc learnin theory might be the path to it.

theory helps u make your own music. and there is more to keys than simply sayin these notes are allowed and these arnt. if u want any details just ask or possible use he search feature, you will come up with loads of stuff.

hope this helps