I started writing my first post before there were other replies. I completely forgot about alternate tunings!
I play with a couple of guitar players who just don't understand the difference between guitar chord shapes and the actual key of a song. The guitarist will say to me, for example, let's learn this song, it's in E.
Then when we get together he's playing in E flat or D. When I tell him that, he says, "yeah, I'm playing in E but I tuned my guitar down."
With five strings, I can adjust for such bone-headedness on the fly without detuning the bass, which can sound terrible if your bass is not set up for it.
I had a similar situation some years back when I was hitting the five hard and was playing with some fellows that would tune down to C. They couldn't get it through their heads that I didn't have to tune down and would pull their hair out trying to convince me otherwise.