Sure. Create a scenario where the season ends with Detroit right at the salary cap. Harris makes $26 million, so when his deal expires it would drop them $26 million under the cap. Meaning they could spend $26 million on a new free agent.
The reality is, they are quite a bit over the cap. Far enough that his expiring $26 million will not create any new space to sign someone. That $26 million still comes off the books, obviously, but it doesn't create cap space to use on a new player. You basically have three options at that point.
Option 1. They like Harris and want him back next year. They have his Bird rights and can go over the cap to keep him.
Option 2. They want to move on from Harris, but still want to use his $26 million on a player. Since letting his contract expire won't create cap space to use, you have to trade him for a guy making similar money. If his contract is longer, you still have him on the roster next year. If he also expires this summer, they would have his Bird rights and could go over the cap to sign him. Much like Option 1 with Harris.
Option 3. They don't want a player and prefer the $26 million just goes away. Duren/Ivey need extensions this summer, Ausar next year. So losing that $26 million off of the cap might be desirable.