I also think Hillary wanted Kaine because he is controllable. She MUST win, I am thinking she is telling herself...because if she does, then she can rule from her suite via Kaine acting in her behalf.
Kaine got the nod because of a deal wherein he stepped down as DNC chair so that Hillary's right hand woman Wassermanshcultz could rig the system for her in terms of superdelegates, voting machines, putting cronies in place, trading out primaries for caucuses, and keeping Hillary apprised of all the developments. In exchange, Kaine was granted the VP spot.