It helps because it signals to the DNC that they will not simply automatically win by default with their shittiest, most rightwing nominees because the other option is slightly worse on a couple of fronts.
It shows that people do not see them as a viable alternative to the GOP unless they actually become an alternative to the GOP.
There’s a reason why Trump won last time and it’s this attitude of entitlement that you’ve embodied which is at the core of this.
If you’re so set upon preventing another Trump presidency then recent history is a lesson for you, or at least it should have been, and attempting to browbeat people into voting for detestable DNC nominees is a failed strategy when you should be pushing the DNC for compromise with people further to the left of you rather than demanding that people further to the left of you capitulate simply because you feel that they ought to become you have a false sense of moral righteousness.
You want my vote for the democratic nominee? Then uphold the values of bourgeois electoralism and earn it.