The security blanket and motivator
Keenum isn’t coming in to take Mayfield’s job, but he can act as motivation to Mayfield. If Baker doesn’t play well, it would be really easy for Stefanski to turn the quarterback duties to Keenum if Keenum gives the Browns the best chance to win. That should get to Baker since Mayfield always loves challenges and plays with a chip on his shoulder.
Keenum also is there for the Browns if Mayfield was to miss time. Football players happen to get hurt and Mayfield is one hit away from missing time. The Browns want to make the playoffs next year. The most important player on the team is the starting quarterback. The second most important player is the backup quarterback.