We went for a breeder for a few reasons:
We wanted a puppy that we could take home as close to 8weeks as possible so that we could have maximum influence on training etc.
We wanted a specific size of dog, and there were a few breeds that we definately didn't want.
The local shelters had few young puppies in, they were all older than we wanted, and nearly all of them were the breed (or mixed with the breed) that we didn't want, or they were males (we needed a female)
I wanted to be able to meet both parents, see their temperaments, make sure we chose a puppy that had grown up with kids and other pets. The breeder we ended up buying from lives on a farm full of animals, with 4 kids under 5, and both parent dogs were family pets and working animals, with the sweetest/most laid back temperaments
The shelters here have very tight restrictions re. young kids, other pets and it looked unlikely that we would pass, although we consider ourselves to be a very good home for our pets