Well, sometimes. Sometimes it's to develop companion dogs. (This could easily shoot off into the old "is breeding for 'companionship' a legitimate purpose" tangent, which I realize not all people agree about... but at least some people breeding them and some people buying them think it is.)
Again, sometimes. Sometimes it's just to sell them.
Again, it gets back to the individual breeder's motivations and practices for me rather than necessarily what they are breeding. Breeding pure bred dogs or designer dogs doesn't automatically put someone into the "for a purpose" or "to sell them" category for me.
To rephrase and elaborate, GSDs are not designer dogs just because they have mixed origins and were bred to be desirable. They were bred for a job and while some may have just bred them to sell them, they were obviously largely bred with the purpose of "furthering the breed." Their breeding was a multigenerational project to create a useful animal. That discounts the breed from being designer as far as I am concerned, regardless of why some people are breeding them now.
I always thought "designer dog" referred to mixes bred to be marketed as trendy. I don't really think of the original doodle mixes as "designer dogs." They weren't bred to be trendy, they were bred to be hypoallergenic service and companion dogs, and dogs were also kept back and there was an attempt (I think ongoing?) to make a consistent breed.
But you have a whole slew of mixed puppies that are only selling for hundreds or thousands because it's seen as fashionable to have an expensive, creatively-named crossbreed. Now, obviously when they bred these dogs they expected the puppies would go as pets, but the majority of these people aren't (to my knowledge) keeping dogs back or selecting breeding stock on the basis of consistent puppies. They breed a litter, sell a litter. I don't think it's a big deal as long as they are breeding sound pups that make good pets, BUT that is a different sort of project than breeding GSDs or a purebred toy breed, etc.