Decorators were introduced in Python a long time ago, in (PEP-318), as a mechanism to simplify the way functions and methods are defined when they have to be modified after their original definition.
One of the original motivations for this was because functions such as classmethod and staticmethod were used to transform the original definition of the method, but they required an extra line, modifying the original definition of the function.
More generally speaking, every time we had to apply a transformation to a function, we had to call it with the modifier function, and then reassign it to the same name the function was originally defined with.
For instance, if we have a function called original, and then we have a function that changes the behavior of original on top of it, called modifier, we have to write something like the following:
def...