Cargo culting OO helps no one, but neither does dismissing it out of hand.
Here are some other things that Adder AA; A=AA.Add(B,C) has over A=add(B,C) that you are glossing over
1) You move the adding logic to it's own file, with other adding-only responsibilities
2) You allow injecting different implementations of Adder - maybe introducing a more efficient one in some cases but not others
3) You enable mocking Adder so that your logic that verifies that B and C are added can be tested without having to re-add B and C all the time.
Not sure how much of a concern this was at the time when OO paradigms were first being created, but in today's world where .Add might be a call to different cloud services, and where unit tests with good mocking are essential for any serious application, these concepts are essential.
Here are some other things that Adder AA; A=AA.Add(B,C) has over A=add(B,C) that you are glossing over
1) You move the adding logic to it's own file, with other adding-only responsibilities
2) You allow injecting different implementations of Adder - maybe introducing a more efficient one in some cases but not others
3) You enable mocking Adder so that your logic that verifies that B and C are added can be tested without having to re-add B and C all the time.
Not sure how much of a concern this was at the time when OO paradigms were first being created, but in today's world where .Add might be a call to different cloud services, and where unit tests with good mocking are essential for any serious application, these concepts are essential.