Guessing it stems from "we need something dead-simple to evaluate that yields a definite yes-or-no answer, with no annoying variables."
I'm trying to think of some other reason they might want a specific moment rather than "pick your own instant within this span", but I can't think of anything. Even if it was to "make sure you aren't claiming the same time on two applications to different places", the person could have simply staggered the applications.