Party A slanders Party B. By the time the lawsuit arrives, Party A has edited his original statement and claims that Party B's screenshot has been edited.

Party A offers something for sale, then after a deal has been done, goes back and edits his original terms for the deal.

Party A enters one of the many contests on this bb. As soon as whatever event the contest relates to is complete, he goes back and edits his entry to become the winning one.

There are many other similar reasons.