We use Proof because it is tied into the project itself so the project is now our electronic job jacket and has all tasks, documents, issues, and communications in one place so as team members get assigned and reassigned, everyone knows what is going on. We can also do a lot of reporting out of Workfront and Proof that you could not get out of Teams. We have also matured enough in some of our agencies that they are sending proofs to clients and/or vendors and proof is a much cleaner/easier experience for that.
It should be noted we average about 500 new proofs a day and 13,000 comments so our use case of Proof might be a little different than yours. But it took us 4 years to get to this point and that is because Proof has been a great tool for us.