[jsmeaton]

Because you’re a python shop that wants some extra safety.

Because you’ve got an existing code base.

Because you want the benefits of a dynamic language with some of the benefits of type checking.

Because the editor experience is way better when you selectively sprinkle in some types.

Because you can choose how deep you go on types depending on the impact.

We have avoided countless production issues just by annotating when something is Optional or not.

[HelloNurse]

Because type annotations are medium to high value documentation for your users.

Because stating the type of something important can expose design issues (e.g. "wait a minute, it isn't always a X! in case Y it can be a Z! Forbid or support?") before they become a serious problem.