They needed to contract to facilitate the trade, but never had any intention of having him play. Saved some money by buying him out, but should have gotten an asset just for taking on the contract to waive him tbh.
Jazz would have been better served having Pleiss come over, but they opted to not start his rookie contract countdown until next season. Perkins would have given the Jazz very little and possibly disrupted the amazing chemistry they've developed. I'd rather they it mess with that. While they might make it to .500, this is still a throw away season. No point in messing with the team dynamic for a one year rental.