VP isn't really a meaningful role unless the President dies or something like that. I get that selecting a VP can have a strategic value, like trying to drum up support from a certain state or certain segment of the electorate, but the VP doesn't meaningfully shape the agenda of the white house unless the President allows it.
Oh I agree it's purely symbolic, but sometimes symbolism is useful.
But, to be fair, I've been wrong on this in the past. I would have put good money on us having a black VP to... ease people into it before having a black president, and before Obama would have said the same thing about a female or gay President but now I'm far less sure.
Again we're like at 99.99% for Presidents/VPs all being old white dudes and it's really hard to get any kind of data from a random sampling of "One exception that we've only had one data point after so we have zero way of knowing if it was an anomoly, a trend, a red herring, or a sign of things to come" so... *shrugs* who knows?
So I could be talking out of my ass. I still lean toward thinking that the VP candidates would be a nice piece of information for people to have before the decision is firmly made though.
The one thing we generally don't talk about when picking candidates and is, in my opinion, one of the biggest factors in how well they are going to pick people for key positions, because it's just one of those things that is very hard to judge from this point of view and point in time.
Who's gonna be Candidate X's VP? Their Secretary of State? Who would be their hypothetical SCOTUS pick? Their thises and thats? Can you see this person being approved by the Senate? These are questions I think there is valid, useful information in that don't get asked of them a lot.
A Candidate who (all other base factors being met obviously) who went "Even though I am in competition with these people for the nomination, I recognize that they offer a lot as evidence by their strong support from other people, so even nominate I will discuss potential roles in my future administration with them if they are open to the possibility" would impress me a great deal.
Sanders, Biden, Warren, etc... they don't necessarily have to be President to do good and be useful.