Sort functions operate in-place
According to php array.sorting,
Note: All of these sort functions act directly on the array variable itself, as opposed to returning a new sorted array
That is, all sorts require an array variable and modify it in-place.
Significance: Chaining
The ability to chain function calls, array lookups, property accesses, and so on is important to being able to write clean, concise code. Requiring the developer to needlessly create extra temporary variables merely to pass a value from one language construct to another encourages lengthy, messy code which is costly to change and difficult to follow.