Sorry for being misleading:
Federal (possibly subsidized?) loans ARE financial aid - which, without completing the FAFSA, she previously had not qualified for (-> clarified, and terminology changed based on @thumper1’s feedback).
However, in our case, the motivation was not us hoping for loans, but because there was another aspect to financial aid: Her research assistant position COULD pay a small stipend, IF the student submitted their FAFSA. Since she is doing the work for that professor’s research either way, she might as well attempt to get paid whatever little stipend might be available.
It was mostly a matter of not leaving money on the table (in the first year of college, we even took out the subsidized, deferred interest loan she magically qualified for (???), which of course, continues to be deferred another 4+ years. During those years, the unused 529 equivalent can continue to appreciate.)