The presence of yield in a function body turns it into a generator function instead of a normal function. And in a generator function, using return is a way of saying "The generator has ended, there are no more elements." By having the first statement of a generator method be return str_in, you are guaranteed to have a generator that returns no elements.
As a comment mentions, the return value is used as an argument to the StopIteration exception that gets raised when the generator has ended. See:
>>> gen = simple_gen_function("hello", "foo")
>>> next(gen)
Traceback (most recent call last):
File "", line 1, in
StopIteration: hello
If there's a yield anywhere in your def, it's a generator!
In the comments, the asker mentions they thought the function turned into a generator dynamically, when the yield statement is executed. But this is not how it works! The decision is made before the code is ever excuted. If Python finds a yield anywhere at all under your def, it turns that def into a generator function.
See this ultra-condensed example:
>>> def foo():
... if False:
... yield "bar"
... return "baz"
>>> foo()
>>> # The return value "baz" is only exposed via StopIteration
>>> # You probably shouldn't use this behavior.
>>> next(foo())
Traceback (most recent call last):
...
StopIteration: baz
>>> # Nothing is ever yielded from the generator, so it generates no values.
>>> list(foo())
[]
