If you have multiple teams, a well structured monolithic application where each module is it’s own service and only exposes functionality via an interface, how does being a microservice help? If each team also writes unit tests, when the other team makes changes, they can easily see if their change affects the other team.

If you are using a statically typed language, you can make a change in the signature of your function and automatically see where the change affects other parts of the code and do and automatic, guaranteed safe refactor.