Because you'd want to know why code does what it does. If a bug pops up, you'd want to know why some code exists and if the original premise of the code is as

Because you'd want to know why code does what it does. If a bug pops up, you'd want to know why some code exists and if the original premise of the code is as it should be or needs revision. I think it's very rare that a developer receives all that information up front in a team. Eg. Old codebase, lost knowledge or people leaving.