Catherine Tate's Character of Donna Noble was meant to be one that would redefine the Doctor's Companions. So far his #1 Companions have been Rose, Martha, and Sarah Jane (yes i know that Sarah Jane was a character from the original Dr. Who show), and all those WOMEN have fallen in love with the doctor had either had to leave because of their love for him or got left behind. All the other Companions, like Mickey or Jack (Ohhh Jack) were really just tag-alongs. Donna is needed to create this idea that the Companion doesn't have to fall in love with the Doctor and that the doctor is capable of just being friends with a women. Plus Donna is awesome comic relief, thought I have to say they have stepped her humor since the Christmas special. Donna would also be an awesome segway into many a main male companion, since on most of the forum which talk about doctor who, they all say that right now if there was a main male companion with the doctors history with companions it might seem like a more...gay over arcing plotline...i mean Jack was awesome but he kinda ruined the idea of a male companion. We now, me, this idea that if there was a male companion it would have to be in a romantic relationship like every other, so far, companion relationship.
Donna is good the Doctor, plus she's awesomely funny.