1) better accomadates a particular vocal range try singing simple man by LS in standard tuning.
LS tuned their guitars down 1/2 step.....another good example....several Neil Young songs....down by the river...only love can break your heart. Even neil on occasion, when his range shifts for whatever reason, tunes his guitar down a half step... the song the Joker by Steve Miller is tuned down a full step.
2) makes bar chords and lead a bit easier to play because of less string tension
3) can possibly extend life of strings? maybe? I don't know for sure, but it make sense.
4) Can always capo up to standard tuning, whereas going from standard to lower tunings requites physically retuning your guitar
1) When asked to sit in with other musicians, re-tuning to standard, if that's what they're using, can be a pain in the ass. Unfortunately, the term "standard" tuning, besides the fact that it is the guitar tuning used most frequently, can play havoc with a musicians ego and/or subconcious. I've heard more than a few musicians, begginners and advanced, speak of "non-standard" tunings as if those who used them were "out of the mainstream" and, as such, were either cheating, or vocally weak. This is easily demonstrated as sheer non-sense when taking a look at the plethora of famous singer/songwriter/guitarists who use, or did use them, routinely.
2) When it's really desirable to capo up to standard, having the capo on the first fret makes playing chords up the neck confusing, because it reconfigures fret markers. Super good players can account for that I suppose, but when I capo up from a half step down, I prefer to capo up to the second fret when possible to avoid these "odd" positionings. If all chords of a song are near the capo (i.e. predominately open as opposed to bar chords), this is not an issue.
3) Does tuning a guitar a half or full step down change how a guitar matures over time? I don't know. It would be interesting to ask the worlds greatest luthiers whether the physics of any particular guitar design pre-supposes that the guitar will be played in standard over most its life.