2.) High ceiling for carries in the red zone
If you watched the Bills the last couple of seasons, you may have seen how they have struggled to not just run the ball consistently but struggled to punch the ball in for six points. Unless it was Josh Allen on a designed run, the Bills lacked production in running the ball inside the 20 yard-line.
Harris brings the ability to be a north and south type of runner for the Bills' running back committee. Harris was seen as the main ball carrier for the Patriots, which included running it inside the twenty.
The possible vision of the Bills running back room is set to have James Cook as the main guy to help stretch the field for the Bills and get into scoring position. Once in the red zone, unless the Bills do a play action, that is where Harris can take the carries and help push the pile in front of him to get a touchdown. It keeps Allen out of the play and allows Cook to still be serviceable as the expected starting running back.