Back when I was a gigging musician, I started off with an extra guitar or two as backups of my main axe. As we grew as musicians and explored different tunings, I would use yet another for that.
Guitars can be pretty fragile things (even my Les Pauls, which may weigh a ton but have well documented weak neck/headstock joints), and I had several literally fall apart during shows. I've had tuners pop out, pots stop working, headstocks fall off etc.
Different guitars have different tones as well. Sometimes I'm recording a song that just calls out for a Strat, or an over-the-top Les Paul tone.
I have to admit, however, that I'm a fan of the instrument itself. I think that they are works of art (albeit functional works), and I enjoy having them around.