Honestly I get why batman has planes, He has to fly around with superman and all the other superheros. But why does he have them? they almost always get totally wrecked. Even his glider suits get trashed. One time he drove the javelin and he made an atomic bomb blow it up. I get he has lots of money and stuff but dang. Sorry I just had to rant. Oh ya and that one time he took the watchtower for a drive XD.
In reality, the FAA would shoot down any unauthorized jet. I mean, can you imagine?
He would shoot the FAA first lol.
The FAA wishes they could track Batman.
To answer the question at hand: merchandise. Mad magazine answered this very question when they spoofed Batman in 1989.
Seriously though, Batman sometimes needs to travel around the world and travel by flight is currently the fastest way to do that. Or he may want to get an aerial drop on the bad guys.