My three wives ask me the same question, all the time. But srsly...Why does anyone have more than one of anything?
If you regularly play with other people I think it's useful to have a spare. In almost 50 years of playing I've never actually had a bass just "stop working". I've had dead batteries in active basses, faulty instrument cable, once had an amp head die, but never had a problem with a bass. But I would hate to have a paying gig, or even just a pre-arranged jam session with friends, and not have a working bass.
And I see instruments as a tool used to create music. And just like a hammer isn't the best tool for every job, a four-sting fretted Jazz bass is not the best for every situation. You might want a Precision, or a five-string, or a fretless, or something with different strings fitted, or ...
But also, a nice bass guitar is a beautiful thing. If you see one you like, and you can afford it, why not buy it? Does it matter that you already have one, or two, or ten, or... ?
