Foster programs are more common in other breeds, like GSDs. The dog doesn't go back to the breeder permanently when bred; she goes back during the heat cycle for the breeding, then it's with the foster family until a week or two prior to whelping; it stays with the pups until they're weaned (or until the pups are sold, depending on the other dogs the breeder has) and then goes back to the foster family until the next time (if any) she's bred. It's maybe 10 weeks a year in two or three years.
Most breeders do not have the time or room for a full-fledged breeding program (and most of John Q Public would call them 'puppymillers' if they did). Fostering dogs, or placing them out on co-ownerships, allows a breeder to utilize a broader genetic base within their own breeding program than they'd otherwise be able to, when limited to two or three dogs.
I'm not personally a fan of flat-out foster programs, because I think they're heavily weighted toward the seller, but I don't think the majority go so far as to be unreasonable, either. True, the buyer gets a free dog with only a few strings -- but my first Boxer bitch was free on a co-ownership and I got a pick puppy back from her first litter (any future litters were mine outright). I'd rather have the owners feel like partners in the process, I guess. Neither view is right or wrong, just different.
As far as the dogs, it depends on what you want in a Boxer. Often dogs from breeders who focus on sports like Schutzhund have a more intense temperament than those from breeders who focus more on the all-around dog; those temperaments are often not well-suited for first-time dog owners, or people who don't plan on doing many formal activities with their dogs. Also, the German Boxer fancy in general does not test for ARVC or thyroid, if those are issues of concern for you.