I've used them for temp. housing on several occasions, they're very handy if something goes wrong and you have no other place to put your male.....like if your new divider wasn't as secure as you wanted it and you didn't have the time right then to fix it, or if something goes wrong with their tank, you can float them in a breeder box in another tank until you can fix the problem.
Its also good if you have tank mates with your betta that take other kinds of food....you can separate your guy out at meal time to give the others a chance to eat, or keep him in there for a few days to fast him if hes eaten too much.
They're also good for Sororities. If you don't have enough time to rearrange the tank right then, put the new girl in a breeder box for a day or two. Or of you have a girl thats being a bully at meal time, or just over-eating, stick her in the breeder box.
Its a very handy little thing to have around, IMO. Many good uses ;)