She appeared there because she really wanted to.

There’s a neat bit of two-part history hidden there:

Part 1:

(Taken from an interview with Michelle Nichols)

Sometime during the original run of ST TOS, Michelle Nichols wanted to step back from playing Uhura. Roddenberry asked her to think about it on the weekend.

During that weekend, she happened to be at some fundraiser, where the host asked her if she could spare a minute for “her biggest fan”.

She said “of course… but hold on, there’s MLK right over there, I’ve got to take the opportunity to talk to him first.”

Host: “Yeah… um, that is your biggest fan. He wants to talk with you.”

MLK told her how much of an impact her role had (for pretty much the same reasons Goldberg mentioned later).

Monday, she rescinded her resignation.

Part 2:

When the staff of TNG heard that Whoopie Goldberg wanted had asked for a minor role, they thought it was joke.

(TNG wasn’t yet the juggernaut it’d become and Goldberg was a top tier Hollywood star.)

But she told them explicitly that she’d been inspired by the Role of Uhura from the start and just wanted to be part of ST.

So they tailored that minor role to her.

To me, she always looks happy as a clam on screen with that.