Someone to fill a role, same reason I was very happy to see Brannstrom off the bottom pair. At the very least, Kleven/JBD/Hamonic are going to play in the defensive zone and play ~1:30-2:00 minutes PK a game. Addison is a guy that has a ~60% offensive zone start over his career, Sens can't hide a guy like Addison, he wouldn't be the guy we put out there in the offensive zone, and we certianly don't want to put him on in the defensive zone. He's played his career thus far playing 3:15 per game on the PP, and hasn't exactly looked very good. Without that, he isn't going to be kept around by teams.
Addison fills zero role unless you're giving him PP1, which he will never get in Ottawa unless all the wheels fall off.
You're not putting him on to protect a lead, not putting him on to go for a goal, not putting him on the PP, not putting him on the PK, isn't physical, doesn't bring leadership or vet presence. Barring 2 injuries that completely derail the Sens season in both Chabot and Sanderson going down, I don't see any reason to want a guy like Addison
The bottom pairing guys should be able to fill 1 of the above roles, otherwise it's a waste of a roster space