Chris Koski makes an important point regarding one talking point aimed at dismissing Biden's Super Tuesday performance: that it's meaningless because he was only winning "red states" that will play no role in the general election anyway.
To begin, it's not really true: of all the states which have had primary elections so far, the state most likely to be "pivotal" in November is North Carolina, and Biden won there by almost 20 points. And in terms of Sanders' victories, it's not like Utah or (in the other direction) Vermont are going to play decisive roles in the general either.
But also, Koski notes, between voter suppression and high rates of racial polarization in voting, the presidential primary is one of the few opportunities for African-American voters in deep south states like Alabama to influence national and Democratic politics at all. Given that they are a core Democratic constituency, it is a good thing that they get the opportunity to make their mark in the primary process precisely because in the general the Democratic candidate won't really have the opportunity to invest any time there.
This is changing a little bit as some shallow south states become more competitive (see North Carolina again). But if the raw bloodless political calculus is that a Democratic presidential candidate really should ignore African-American voters in Alabama during the general election campaign, that makes it more important that they have the opportunity to exert meaningful influence in the primary.