Most of the time I get a call to rekey locks is when someone purchases a new home or property. This is to ensure there's no keys floating around out there and anyone who previously had a key can no longer use it to unlock the doors. Another common reason is someone has a bunch of different keys to different locks and they're looking to simplify things a bit. This is the best and most affordable way to change the locks without needing to purchase all new hardware, which can get very expensive.
What I do is disassemble the locks and dump the cylindrical pins inside the plug, then cut new keys to a completely randomized code and insert new pins to match that new key pattern. Amazing isn't it?