The goal is just to narrow down the search area after a crash. Even at 700 MPH you can only cover 175 miles in 15 minutes. That's a pretty tractable area to search for a blackbox ping. And that's if they have no other info about the crash.
100K square miles is a pretty big area to search. Actually more than that since a plane cruising at 30K feet could glide quite far if it maintained some airframe integrity.
