Why is Heck Tate significant to To Kill a Mockingbird?

I agree with the previous posters. Heck Tate's role in the novel is connected mostly to two scenes, the shooting of the dog and the trial scene. In narrating the trial scene to us, Scout even conflates the two scenes. In the paragraph in chapter 21 beginning "But I must have been reasonably awake," Scout makes a number of clear connections between her father shooting the dog and (I suppose) her hope that her father will be equally successful in defeating the jury's racism.

I don't agree with the posters, though, that Tate is the embodiment of justice or compassion. He certainly does represent the law in the town of Maycomb, but he doesn't enforce the law equally and impartially. He purposefully hushes up a murder because he doesn't care one bit for the person murdered. That's not really justice, in my view, at least not the "blind" kind that's held up as our ideal.