I bought a new Roland FP-90 a week ago. It is a very nice piano.
The thing is, I just use the default grand piano sound and add some ambience to it and I'm all set. I don't seem to need anything else.

Am I missing something important here?

After 1 week, not at all. After longer you might want a variation or change, then you canexplore further. Of course you could do that now and have a wow moment, or not.

Why would you need anything else?

If it was a wooden piano with strings, that'd be your lot! The variety should be in the music, not the piano tones. We're all crazy here and don't much like practising . . .

I think many of us just use the default sound. I suspect it is usually the best piano sample the manufacturers have.

If you like the sound no need to change it. Enjoying playing the Piano!. But if you get bored of it, or want to mess around with other sounds or do splits e.t.c you should have no problems doing that either.

Just enjoy it!