For both reasons, really. My mom is a special education teacher, my uncle has been in a wheelchair since before I was born, and he is part of a group to help wheelchair-bound people become more independant and learn that being in a wheelchair does not limit you, but instead challenge you to find other ways to accomplish things.
I've known people with service dogs of all kinds--assistance dogs who are the hands for those who can't pick up what they drop, or have trouble giving a clerk money to buy simple things, or have trouble opening doors, washing dishes, doing the laundry, etc....guide dogs who lead their handlers through the safest routes and grant them independence they've never had before.
For me, owning a dog is a simple pleasure; wonderful companionship in the form of a furry friend. They do not judge, they do not question, they do not hold ill will. They love purely and deeply, no matter the circumstance. For me, they are the best friend a person could have and also make life more interesting.
But for someone who
needs them, literally needs them to function....they are so much more. They are a friend, a shoulder to lean on, sometimes literally. They grant them independence they may never have had before. They allow them to see the world, literally and figuratively depending on the disability; they allow them to socialize and become a member of society. They allow them to live. To be able to give that kind of a gift to another human being is nothing short of incredible. Training the dog is fun, definitely. But it's more than that. You are changing another person's life in so many ways, simply by training a dog. When I am actually able to feel what it feels like to give someone that, I am sure there will not be words that can describe it perfectly. But I know it must be incredible.
So, it's both. Training the dog, and helping the person. Yes, I think it would be nice to use another breed, because I do think others could be suitable for it. But, I will not put that personal desire over the needs of those who need these dogs.