Is there any significant downside to a pitcher doing his “side session” as a relief appearance? IOW-2 days after his start, he’s available to pitch in relief.
Pitchers are usually focusing on a particular pitch or location in their side session. An in-game situation may not accomodate the session’s focus. He might want to work on throwing fastballs low and away to right-handed hitters, which wouldn’t make sense to do 15 times in a row during a game. During a side session, a coach can help tweak your mechanics, if necessary, which is not ideal during a game. Also, I doubt pitchers are throwing 100% in side sessions between starts.
I think this is they key part. Both mental and physical effort are much higher in a game situation than a side session.
You also can’t count on having a particular number of pitches with a particular pace like you do in a bullpen session.