Alot of inspiring songwritters and composers do it because it's the best way to learn how to compose, especially if you want to learn why a band sounds so good. Usually it's done by buying complete sheet music (bass, guitar, voice) and then copying it to powertab or something. Drums are pretty easy to figure out. Advice to anyone who really wants to learn, figure out each part by yourself and use a midi sequencer or a notation software. Midi is an easy check yourself too because it plays back and if something doesn't sound like the original then you know it's wrong.