Why wouldn't one compare apples to oranges?

Assuming that the saying “It is like comparing apples to oranges” means “comparing things that are impossible to compare”.
Is comparison not by default researching two inequals?
I mean, why would I want to compare an apple to an apple (or a BMW M3 to a BMW M3)...., nothing spectacular would come out of that surely?
So, does the saying “It is like comparing apples to oranges” makes sense or am I on to something here?
If you can say something funny about comparisons, be my guest, but I am also looking for a ‘real’ answer.
Thanks!