In my opinion this is aimed at Kyren Williams as the third down back. Sony is one of the best pass blocking RBs in the NFL. Pass blocking is a skillset few RBs possess. Also, as I've been saying I think they are going to emphasize the inside run attack which favors power over long speed. With Evans and now Michel they have power RBs in their top 3. As I've said prior a true 3rd down RB was missing from their RB room.
I still see Evans and Akers rotating on early downs but Michel should be on the field on passing downs. Akers is 5'10", Evans is 5'11" and Michel is 5'11" with all 3 well over 200 lbs. Williams and Rivers are both 5'9" and under 200 lbs. Your first 3 RBs should project as starters and Michel now gives them that. All three are good inside RBs with enough speed to take it outside. All three run with power.
In the Super Bowl, starting inside the 5 yd line, Michel got them out with one inside run getting out close to the 25 yard line giving Stafford room to work. That was a key play that helped win them a Lombardi. I was disappointed when he was not brought back. Bringing Sony back reinforces what I've felt about the Ram's run attack this year. It's going to be all about simply moving the chains with the big plays coming off of Stafford's arm. It's why I said a few days ago the Rams aren't looking for another Gurley, their offensive philosophy is now grind it out on the ground taking deep shots downfield.
That is not to say their RBs aren't capable of ripping off a big run because they can. But IMO it's going to be all about staying ahead of the chains with their run attack. That's the kind of run game that sets up the play action, i.e. an inside attack with power RBs. It also, IMO signals that they are ambivalent about extending Cam after the season.