DONNA!! WHEE!! (I love Donna - but you all know that already)
From "The Fires of Pompeii:
The Doctor is used to saving everyone. Or at least, that's his goal: to save everyone. But sometimes, that just doesn't happen - for all of the Doctor's best intentions. So what does he do? If he can't save everyone, then might as well make sure everyone dies. Why not? He's a Time Lord - he's seen planets and galaxies die. He even witness the death of his own planet and his own people. What's one more insignificant city on a backwater planet?
But Donna is not having that. Not at all. Earlier in this episode, she asserts her own authority over the Doctor - simply by being a human. She thinks ought to have a say in what happens to people on her planet, even if those people died centuries before she was born.
That's not to say Donna is arrogant or smug about the situation. Far from it, actually. Beyond being a human and caring what happens to people on her planet, Donna really gets the Doctor. She understands that he feels guilty about what happened to his own people. On one level, she even understands the Time Lord rules that he still follows, even with him being the only one. But she also believes that he can be better than all that, if only he would let himself move past the bad things he's witnessed and done - including how things with Rose and Martha ended. Donna doesn't want him to bury himself in his grief. She spends all of Series 4 bringing out the best in her friend - and this scene is really the beginning of that.
While "Partners in Crime" showcased how much fun Donna was as a companion, "The Fires of Pompeii" shows that there's more to her than being comic relief. Donna keeps things in perspective for the Doctor. Even if he can't save the whole town - he can save somebody. That's a small victory, at least.
Heaven only knows how much help she would have been on Mars.