I think the reasons "she" picked him are simple. (btw she had no part in this decision)
1. They needed a grey-headed white guy for optics, to counter and win over the systemic racists everywhere.
2. They knew she would have a decent chance of losing and didn't want to waste one of the tiny handful of Dems who are semi-capable of the office, and who might be electable in the future.
3. They needed someone who fit 1 and 2, who was also dumb enough and committed enough to the party to believe in her, who could keep a straight face in 'selling' her to the folks.
Boom, Walz is your guy. Two throw-away candidates who in the big picture view, have he value of a crushed up 7-Eleven cup on the side of the road.
Pretty sure most of you don’t know me.