Possibility for shepherd in there, but hard to tell on a younger pup.
DNA tests - it all depends on the test, the collection, and the breed database the company holds. I doubt ANY company has a "complete" breed database, but some have more than others. If the breed is not in the database, it can't be detected, plain and simple. If the collection is bad, its going to mess up the results too.
And as far as how important it is about breed - some breeds have characteristics that others do not. Some breeds respond better to training than others. Some breeds have behaviors that other breeds do not. Some breeds are more prone to certain health issues than others. Mixed breeds will have a combination of all breeds that make them. Knowing what those breeds are, or at least the dominant ones, is important because you know what to expect. You will know what to expect in the dog's behavior, training or trainability, intelligence, stubborness, and health. By knowing that my dog is part American Staffordshire Terrier, I can recognize she could have a tendency for being dog aggressive and having a high prey drive, thus I know to be careful with her around other dogs, not let her play too rough, and watch behavior cues to stop a fight before it happens, and be careful with her around small animals. By knowing she is part Pointer, I can also recognize that she will want to scent and track, I will know what she is doing when she's pointing. I can also alter my training methods to suite her better, because both breeds can have a tendency for being somewhat stubborn at times. By knowing she has Pointer in her, I now know why she is suspicious around strangers, since pit bulls really should not be. She isn't a "bad" pit bull, she is a "good" pointer, in that respect. Because she has inherited characteristics from all the breeds that make her, I now better understand her. Does that make her any different as a pet? No, it really doesn't, but it helps enhance MY understanding of her and how to deal with her.