It seems to just give a heads up at the time you request. So I just set one for 1:45 PM with a 5 minute early reminder. Here's what it looks like:
View attachment 2214632
So it's essentially just creating a separate notification before your reminder is actually set, so I guess kind of a second reminder? I can see how it'd be useful if your reminder is a meeting but then that would more logically be on the calendar than in Reminders, I guess. Or if I had a reminder to go to the post office at a time, that could be useful I guess.