What motivates or has motivated me:
1. I need something different for a gig. In one case, I was in a band with a bagpiper

who used two sets of pipes, one pitched in G and one in D. They were never in tune with each other. After getting tired of retuning multiple times during a set I decided to use two basses, each tuned to one set of pipes. G and D pipes mean a low D is useful so I was already using a Steinberger L2 which could be downtuned without needing to make any neck adjustments. When a Steinberger XM2 showed up for sale cheap I bought it. Before each set I checked the tuning of both pipes, then tuned up the two basses. A year or so after that gig ended, I sold the XM2.
2. It's just cheap and cool. Like a Danelectro Silvertone for $65, a Supro/Airline Pocket Bass for $250, that Steinberger L2 for $550
3. A backup...when I had only one bass, it was worth getting a second one for that purpose.
4. Wanted to try something different...fretless, 5 string, 6 string.
5. Different setups for different gigs, like one BG with flats and one with rounds. I use two DBs, one strung with synthetic gut strings and one strung with steel plus a EUB that's used for travel when there's no room to carry a real DB.
I'm not a collector because I don't have any rhyme or reason to what I own besides utility for gigging.