Not sure what you're comparing OO to , but it certainly makes l little sense if you're comparing with FP. Grouping by function is so much more natural than artificially attaching the same function into different objects. When I copy, do I care whether I'm copying a document or a program? All I care about is the functionality, which is copying. This applies to many cases. OO design forcefully fractures every function into different objects, which makes extension a nightmare.