She's an actress and a comedienne, and in my opinion good at both.
But, besides that, she was in Doctor Who because she had previously starred as the grandmother she mentioned in the Martha series for the two-part Human Nature/Family of Blood story. I guess the granddaughter device was used because the original character wouldn't have reacted in a way suitable for a quick two-minute scene.
I guess she gets promoted above other guest stars because the Doctor-as-a-human loved her and the two episodes were highly regarded by critics, fans and casual viewers. I'd imagine the only other similar candidate is Madame de Pompadour, but the Doctor had already jumped in and out of her entire life so probably can't go back. And Sophia Myles is off in Hollywood, I think.
EDIT: and it's Jessica Hynes now. Has been for a few years.