1. help the singer hit "higher notes".
2. sound "heavier" and just that "bit off" that half-step down tuning gives.
In conjunction with "drop-D" tuning (pulling the low E-string down a full step in order to give a "deeper", more heavy tonality), this gives much of modern "heavy rock" heavier feel.
My first encounter with half-step down tuning was, I believe, Motley Crue's Too Fast For Love album, where I beleive Mick Mars used both tunings in order to use a beer bottle for full chord slides (dropping the low-E allows barre chords, the staple of rock music, to be played using just one finger or, in Mick's case, one beer bottle) and that exceptionally low "drop-C#" (remember, we dropped the low-E down a step and a half to C#) drone growl that gave that album an exceptionally hard feel.
[edited]
I can easily say that in the 100's of rock songs that are tuned a half-step down (the entire Guns N Roses catalog for one quick example) and thus lend themselves to keys like G# and D# instead of A and E, that I have almost never encountered a song in G# or D# that were not using half-step down tuning.
This is not likely to happen with a recording of the instruments I’ve always played (piano and trumpet) but might happen if I’m listening to guitars or organ.
On the other hand, if you do have perfect pitch, you can lose it over time! I've had perfect pitch all my life, and now that I'm going on 40 I surprise myself by sometimes hearing a song a half-step off (i.e. I'll assume a song is in F but it's really in E). This has never happened to me until the past few years. Not sure how much is due to my ears aging, and how much due to I'm not a practicing musician anymore.