The relation to parts to function is more complex in a biological systems than in an engineered systems, but I agree that they sometimes have have parts with understandable function, for example the heart pumping blood. These doesn’t happen always, but it happens enough to require an explanation. Why should evolution make parts with narrow functions, when it can also make integrated functions that emerge holistically, like the immune the system? I think the answer has something to do with what is easy to evolve. I think the same modularity that makes organs understandable also makes it easier for them to evolve under natural selection.