alternate tunings question

As you may have guessed from the subject title I have a question about alternate tunings. I can understand dropped-d or dropped-c tunings or open tunings or any tuning that changes the intervals between the strings, because it could makes songs possible or easier to play. But i'm confused about tunings where each string is dropped down by an equal amount.

Jimi Hendix, Van Halen and Slash, among many other rock guitarists often tend to tune every string down half a step to Eb Ab Db Gb Bb Eb. Why is this such a common tuning? I can understand it if the song was in the key of Eb and thus required the low Eb but this rarely the case.

I once read that tuning down half a step makes the guitar sound dirtier and thus was an important element of Hendrix's unique sound. Why is this? I also heard that Slash tuned down for November Rain to make it easier for the singer, but why not just play all the notes one fret lower? The fact he tuned down in so many songs means this is unlikely.

Also, why is it that when guitars use this tuning the chord names and keys tend to remain the same. Take Sweet Child O'Mine. When played in standard tuning its in D. However Slash tunes down so it is actually C#. This would seem more awkward for the singer. Also in the Powertab, version even the bass is tuned down, which I thought wasn't possible, although im proabably wrong because I don't know much about bass. But the strange thing is that in the notation for both these songs is as if it was in standard tuning. A note at the 12th fret on the fourth string for example is shown as a D in the notation even though when tuned down it is actually a C#. Is this for simplicity, leaving the notation as if it was in standard tuning so it is easier to read? Going back to November Rain, I'm pretty sure its in C but according to any tabbed version its actually in B, when the alternate tuning is taken into account. Can someone confirm which it is?

Thanks for any help