I don’t think that any votes for a candidate based on the VP per se, but the choice of VP can reflect on the thoughts and priorities of the candidate themselves. A pick of Warren for example would indicate that Biden might be willing to take up some of the progressive movement’s causes.
I think Harris is a good way to thread the needle of the BLM movement and protests. Picking a black women certainly indicates that he is not going to ignore the concerns of people of color, while picking an aggressive former attorney general should blunt any attacks that if elected Biden will abolish the police. Such attacks are completely baseless of course, but might work on low information swing voters swayed by scary ads.
He will definitely go there, not because he is desperate, but because he is a racist asshole who never passes up a chance to punch down.