I'm completely new playing the guitar and was wondering if someone would mind answering this question. I've just bought a greenday guitar book and on most of the songs it says to tune down half a step, so High E would become E flat etc. My question is why does Greenday do this, is there a benefit to tuning down half a step. I can't understand why they do this.

Many Thanks

they're looking for a heavier sound. thicker, chunkier etc.

if you're having trouble tuning to Eb, the tune to standard and then tune everything to the Estring 6th fret.