Why it was not written like this :

Question

operation=multiply() instead of this operation=multiply ??

Answer 1

operation = multyply() => this is the syntax to call a function.

operation = multyply => this is the assignment, assing right side variable value to left side variable, in this case memory address of the multyply function.

Answer 2

The difference is, what data type operation has after these lines.
The first line executes the multiply function and stores the result in operation.
The second line does not execute the function but stores the function in operation. So after that second line you can call operation() as a function.

Answer 3

can you show the code where you see this?