You can set up a single seat alert for free, meaning a flight where you don’t have the seat you want they will keep checking for better seats and ping you when one becomes available.
Say you are a united elite and there’s no economy plus seating available when you book. Or you want an exit row on American or Delta. Seats open up when someone cancels, or most likely close to departure when another passenger’s upgrade clears.
Expertflyer lets you know when the seats you tell it to check for become available, and you can then go online or call and change your seats.
Often the best seats are gone close to departure, I booked seats to Dallas a couple of days out a few months ago and was in a middle seat towards the back of the plane. I traded up to an aisle and then to an exit row as travel approached thanks to Expert Flyer’s seat alerts.
Expert Flyer subscription (paid) members can have multiple simultaneous seat alerts. The app also includes access to SeatGuru seat maps with recommendations and warnings about specific seats.