)
They've been trying hard to paint Biden as pro-antifa (he's not), or supportive of defunding the police (he's not), or such. Harris' pick means such attacks will fall extra flat to anyone who reads more into a person's record past the slogans. And to the ones that don't will just as likely see her pick as promoting equality as much as those who see it as an attack on whites.
Basically Harris' status as a minority who supports minority-hating "tough on crime" measures let's Biden have his cake and eat it too owhen it comes to flanking many of the Republicans' race issue talking points on race issues.