You could be sorry giving up your confirmed seat that day.
Please understand that the seat map on aa.com only shows assigned seats, not purchased seats, many times there is a significant gap between the two numbers. There is a good chance the later flight will be overbooked (it may be already and you have no way of knowing that). If that flightt is overbooked, standby is very iffy.
Or, what if another flight, on another airline, cancels and they book seats from AA? (This has happened to me more than once, even had a boarding pass taken away after it was issued for standby.)
AA will have no responsibility to get you to you final destination if voluntarily give up your confirmed seat.