Before creating accounts for the developers and giving them access to the TM so they can review required countermeasures and update architectural design if needed.
If I am handing off the TM to a coworker due to planned leave I version what I had done
As you mentioned, also version before major changes in architecture
I also find it interesting to be able to go back and see how the architecture changed over time and through multiple workshops
New Review New Version - Possible that you want to be able to demonstrate difference between the last engagement or audit and want to show them dashboards at two different points of time.
Major Changes - as the application changes, you could version the threat model over time to match your product or deployment versions for when it was threat modeled.
Additions - as you start the threat model, maybe you only threat model the front end vs backend. V1 might start with all public facing components and then v2 might add in backend components, v3 might add in third party integrations.
Wanting to try something new in the product - since the diagram can always be reverted back to previous versions, you could use as a way to test out new components or structures.