When colleges get students off from the waitlist, do they admit students who have higher stats first? Or they admit any students just to fill up the spots?
In general, schools admit based on institutional needs.
I love to refer to this article which is posted specifically for Georgia tech but has a universal message:
This should be pinned at the top of cc for the next few months.
Many colleges are more likely to admit waitlisted students who have expressed a continued interest in attending there.
But what if the colleges that waitlisted me do not accept any continued interest letter or appeals?
You opt into their waitlist and move on to the acceptances you do have.
Then you don’t send in any. Opting in to the waitlist is sufficient demonstration of continued interest for these schools.
As @Gumbymom said up-thread, waitlist decisions are driven mainly by institutional need. Writing a LOCI typically doesn’t make much difference.