Leaving aside code documentation (aka dockblocks), I can think of dozen of examples where you MUST comment you code..

  - strict order of execution
  - math formulas in the code (or its results)
  - external systems requirements/hacks
  - non-explicit side effects
  - legacy or costly known issues
  - intention of cryptic code
  - design choices (and its tradeoff)
  - ..

This obviously implies you actually care about who will maintain such best