[swish_bob]

Why it exists ought to be explained by the existence of a test.

This isn't always possible, but it's far more possible than many developers seem to think.

[minaa-chan]

That just makes two things that need to be commented doesn't it?

When I read code I care about what the code is supposed to do and why it's supposed to do that.

There is code that passes the current tests and is logically sound but no longer fits the business requirements. Knowing that it was implemented to solve a certain use case helps the reader / reviewer see whether the code and the test still fit or if they should be updated.

The best comments I've come across comment the intention and any context that isn't immediately obvious from the function.

[swish_bob]

Your test should explain what it's testing. Via it's name or some other mechanism. Otherwise how do you know what's gone wrong when it fails?

[SketchySeaBeast]

Does that explain why, though? It explains that it's there, and someone obviously wanted it to be, but it still doesn't get at the why. I've found that most of the logic I'm left scratching my head at is due to some business rules that there's no better way to deal with - even if a test exists for it I still can't figure out what the tests purpose was, other than to just verify that it's working.