To me it's two hobbies linked by a common theme.
One hobby is playing bass in a band and when gigging I tend to settle on one bass and stick with it.
The second hobby is tinkering with basses. Buying quirky or old instruments and reviving them. Setting them up to play their best, restoring or refinishing and modifying them.
I have quite a few basses (and guitars, shhh) and while they are not an investment as such, they have a monetary value, if I need that money one day I can sell some or all.
It's much more fun owning basses than it is owning money. A money collection is the dullest thing I can think of. Money is just an IOU, much like my instrument collection. Both can be traded for other stuff when the need arises.
If I had to own only one of mine though it would be my modified Variax. They don't get much love but it has everything, playability, looks and versatility.