On the majority of nights, it makes more sense to start Morris with Horford at Center.
This allows Baynes to come off the bench at Center and anchor our defense, rather than Morris or Theis, one of which would have to play out of their natural PF position.
Also, starting Horford at C beside Morris simply gives us a heck of a lot more space for Kyrie, Tatum, and Brown to carve up the paint and get to the rim. Get us out to an early lead.
Our defense and rebounding are already top of the league, there's not tons of room for improvement. On the other hand, our team could use more offense, if anything, and Morris in the lineup is a big boost.