[Python] Why do we need wrapper function in decorators?

If I create a decorator like the following:

def my_decorator(some_fun):
def wrapper():
print("before some_fun() is called.")
some_fun()
print("after some_fun() is called.")
return wrapper

@my_decorator
def just_some_fun():
print("Wheee!")


Another decorator can be defined as:

def my_decorator(some_fun):
print("before some_fun() is called.")
some_fun()
print("after some_fun() is called.")

@my_decorator
def just_some_fun():
print("some fun")


What is the benefit of using "wrapper" function inside decorator? I didn't understand the purpose.

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