Comment by cema

I think another exception (in addition to the already mentioned help with an otherwise difficult syntax) is when comments are used to visually represent logical segments of the code. Some style guides suggest to write these in all caps. Eg:

  # CONFIG
  ...
  # LOCALS
  ...
  # EDGE CASES
  ...
  # REFORMAT INPUT
  ...
  # MAIN ALGORITHM
  ...

Etc.