When to write a function

It's never necessary. It's just good programming style.
I need to do many simple but related operations one after another to perform one complicated task that has a well defined result.
I need to change the algorithm I have hard coded to the program but I know that if I erase it I can't get the program working again.
I need to add together two numbers and I repeat that many times in my program.
I want to test a piece of code separately from the other program.
I need to complete two unrelated operations with the same data.
I need to repeat the same procedure with different data.
Two programs need to do the same kind of calculation.