It has to do with how you need to use the information returned by the data page.  For example, lets say the data page returned a list displayed as a repeating list on the user interface.  If the user was to have the option of selecting rows and actions occurring on that row, it would be useful for the data page and it's resulting page list to be editable to capture the user input.  And later processing can then copy or use the information captured.

Tara, self study support team

One specific use case for editable data page is editable data page with requestor level scope can be used to copy data between multiple threads.