That said, you're asking us why we have more than is necessary. Here are my answers:
- Variety in appearance: I have conservative and flashy basses, each one in a different color.
- Variety in tone: different pickups, different body woods.
- Variety in setup: I have flatwounds on one, rounds on others.
- Vanity probably factors in as well... but I only have 5 basses whereas some have dozens.

All I play are five string fretteds, but (as I'm sure has been mentioned), others like to switch between fretted and fretless, four/five/six strings, even active and passive.
P.S. Congrats to you for being satisfied with just one! Again, I don't think you really need more than one (aside from the backup).