Any code you have not looked at for six months might as well have been written by someone else.
Not a joke about other people's code. The six-month version of yourself has lost the context that made the decisions legible, which is the actual case for comments explaining why rather than what.
In practice
- An engineer reopens their own module after two quarters and cannot reconstruct why a function takes four arguments.
- A researcher returns to their own notes from last year and cannot tell which figures were final and which were working.