Can not understand what original designer(s) intent / thought process was unless there's documenation (physical and/or comments). As software complexity increases, using educated guesses / testing becomes more difficult to do in way that doesnt' introduce other issues. (vs. relevant documenation / explaination). Lot of times easier to start from scratch than do guestimate thinking/approaches.

NASA has a long term/historical projects with public documenation about supporting long term project issues.

TX company that still uses punch cards (2013): https://www.chron.com/news/article/Conroe-company-still-usin...