Comment by EraYaN

Because the why can be completely unrelated to the code (odd business requirements etc). The code can be known to be non-optimal but it is still the correct way because the embedded system used in product XYZ has some dumb chip in it that needs it this weird way etc. Or the CEO loves this way of doing things and fires everyone who touches it. So many possibilities, most technical projects have a huge amount of politics and weird legacy behavior that someone depends on (including on internal stuff, private methods are not guaranteed to not be used by a client for example). And comments can guard against it, both for the dev and the reviewer. Hell we currently have clients depend on the exact internal layout of some PDF reports, and not even the rendered layout but that actual definitions.