Speedy Comprehension
When you're working with code bases in the tens of thousands of lines, speedy comprehension is often necessary. Two things really boost comprehension: input type and output type. What am I getting, and what do I need to produce. Not really obvious when you're working in a dynamically typed language such as Python.
Proven Documentation
Sure type annotations and doc strings exist and they can work, but they are not enforced nor are they verified. When you introduce a static type checker such as mypy, what you get is proven documentation. You know what you're getting and what you need to produce - input and output - isn't that the focal point of most programs!
So what does that look like?
Something like this:
def greeting(name: str) -> str:
return 'Hello ' + name
This is a trivial use case because the code is only two lines long. But imagine you are scanning through hundreds if not thousands of lines. Trust me, types help.
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