what is this doc page for

[micseydel]

what can these be used for?

They are syntax for wrapping a function definition at definition time. They're not for changing built-in / third party functions at runtime. They're logically equivalent to other forms of wrapping. The best example I've seen is caching / memoization, and it doesn't necessary make the code shorter, though it helps with separation of concerns and if the decorator is reusable then you can have less code.