To be fair, we don't know that this is the case with Clara, we haven't really been told what the explanation is with her. You also can't expect every character to be bog standard normal, uninteresting, and of not particular significance, because there wouldn't be much story if that was the case. It's better that the companions are sometimes woven into the plot, rather than just being a mechanism to allow the Doctor to explain things to the audience (which is always one of the reasons for the existence of a secondary character like this).