Need of return in function if statement inside the function already returns

[Athul]

Yeah I know that. What I meant is
When I run below code

a = (10, 20) 
sum(a) == 20 or a[0] == 20 or a[1] == 20

It gives the answer as

True

or

False

That means it returns something.
SO when I make this as a function, why do I have to type return before

sum(a) == 20 or a[0] == 20 or a[1] == 20

this, since it itself is returning the answer