He also clears the way for a young up and comer to get a head start on holding that senate seat as VA moves further blue.
He isn't a big threat to mount a serious 2024 bid either, giving the party a fresh face if they have a crack at five in a row.
And he's made for the VP job. Get in line with the POTUS, not cause any waves, and be proficient with completing the second tier of tasks.
Warren would be wasted as a VP. Booker can build more of a resume in the senate. Sanders wasn't really an option but if Clinton wins he gets to be chairman of the congressional budget committee so he wouldn't even be interested. Vilsack is another brand of white bread. I'd have found the Stavridis pick interesting or Perez but both give up the no public office experience attack on Trump and both bring a one dimensional resume.
I think Kaine will acquit himself far better than many in here expect. He checks a lot of boxes and has been incredibly well liked by every constituency he has represented. Pushing VA completely off the swing map,would also require Trump completely run the swing state map, a near impossibility.