It's a habit that goes back decades on e.g. web forums and editable comment sections; some forum software automatically marks a post as edited, sometimes adds a percentage how much was changed, and leaves an edit log for moderators. They add form fields so people can indicate what they edited, or add a grace period (setting) that avoids adding the 'edited' marker if edited within a set amount of time after posting (for e.g. typos).

TL;DR it goes back a lot farther.