He is the right kind of actual Democrat who supports the policies Kamala supports - he's got midwestern dad vibe energy, not a more 'educated snooty coastal elite', or whatever bullshit is that plays so well against Dems - so he is the right messenger. Beshear was the only other one that could have pulled it off. Kamala has already been painted as a California liberal elitist from four years ago, and her poll numbers and fundraising have done nothing but change the game. The communicative vibes of Walz are very very good and again, they come from that midwestern dad vibe, he is the guy you want to have a beer with, since that matters to moderates.
He will be an effective messenger, as pointed out he already came up with the biggest messaging narrative of the campaign so far with the weird bit, and he hasn't even started yet - and he has the skills to combat attacks on him.