Basically, direct recorded bass sounds kind of flat. While you will get decent tone, and the bass can sound “good”, putting it through an amp+cab sim will make it sound much more natural,less two-dimensional, like it was coming through a real amp and speakers. I actually think they sound better than miking real amps, but that’s just me.
Taking the songs I posted as an example, they sound quite different and not nearly as good if I render them without the amp sims. Particularly the guitars, which sound kind of limp and dead in comparison.
Compare this:
to this:
The bass and guitar don’t sound bad in the first example, but they sound much, much better in the second.
Many amps - particularly tube amps - really add their own color to the tone. Lots of these like the SVT have very distinctive, sought after sound. A slightly overdriving SVT is an absolutely iconic bass tone. And sims get you that, at a fraction of the price, and probably recorded better anyway.
Here’s the isolated clean bass:
Sounds nice! I do love how my TRBX sounds.
But here it is with the amp+cab sims:
and that’s just so much better. Very characteristic tube amp overdriving sound in there, I absolutely love it.