I think the main reasons to use a preamp are:
1) the pickups are weak (but I have not found that to be a problem even with my Danelectro; just turn up the amp!)
2) the bass came with a preamp that is integral to its characteristic sound (eg. Alembic ... or maybe the old Music Man basses)
3) the preamp gives you some control that you like having at your fingertips always, and that you wouldn't have without it.
In the latter category, I still have my much-abused Ibanez blazer with added Alembic Activator pickup & preamp combination. The tone control on these is unusual - it's a slightly resonant lowpass circuit and the knob adjusts the cutoff frequency. More like the "filter" slider you see on a lot of old analog synthesizers. The resonance is not adjustable (just as well, I'd probably just f*** things up).
So the bass is a bit of a chameleon, and turning the knob changes character quite significantly, while always preserving lows.
Still one of my favorite one-knob tone control designs ever. But I'm not sure it would work well with a pickup that already had a strong personality. Would have to try it.
I rarely if ever turned it DURING a song, so technically I could have got similar results with an outboard tone control, but I still love that bass and that knob is an important part of it to me.
All of this was a happy accident. I wasn't happy with my sound (in retrospect, I blame my shitty Peavey amp) and kept trying different things. I bought the Alembic pickups because I was a huge Entwistle fan and he was playing Alembics at the time. I had no idea I would come to love that tone control.