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.
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