The reason I like Tillman for this group is his physicality. I don't really see us having many good defensive match ups (not named 39 year old Al Horford) against guys like Embiid, Giannis, Jokic and other big physical players 4's and 5's. Tillman plays a bit like Grant Williams in this regards and that is valuable come playoff time. At the same time he has the ability to guard threes and twos on pick and roll plays, much like Grant did - for example, he played some nice D on Doncic when we needed it. He brings a defensive physicality that we generally lack in the frontcourt.
Certainly, we will survive without him, but for the right money he brings a very valuable defensive component to our team - I hope we re-sign him.