-- Biden already has California locked in so he doesn't need Harris for that purpose;
-- Perhaps they think Harris is the only one "tough" enough to take on Trump. Problem is, she won't be taking on Trump, she'll be up against Pence. Harris's nasty and brutish demeanor coupled with her grating voice will be quite a contrast to a gentleman like Pence and won't "play well" on TV.
-- Or perhaps they think questions about Harris's qualifications for President (I expect that in the extraordinarily unlikely event Biden were to win the Party would have him removed before dinner that day) will provide a distraction from the Democrat/Left's platform of insanity. That won't work either as her qualifications (as was the case with a former semi-black Democrat/Leftist candidate) don't matter now that John Roberts and the USSC have ruled that Constitutional provisions don't mean what's written (e.g., governors may suspend the first amendment if they have a "good reason"; mulatto privilege precludes any questionning of qualifications ... lest you be called a racissss by the NYT and the Washington POSt.)
So while I don't understand the reasoning, the choice made on Biden's behalf by the America/American-hating Democrat/Left Party leaders of the newly botoxed Harris appears to be a good thing ... for Republicans.
I think he picked her to obtain the minority votes in the battleground states... fail