I expect it is to see what Niemann has to offer. He had a 1.5 era in 4 games this ST and is 1-1 with a 3.27 ERA in AAA. Garza on the DL and with Kaz taking his time back, this is Jeff's turn to show what he has. Hammel is out of options pretty much, and the Rays can't afford just to dump him through waivers...need all the young arms they can have.
When Garza comes back, Niemann will probably got back down; unless he pitches incredible or Hammel just bombs his next few starts.