It helps in strawman cases when users have the data, and the platform doesn't have social features. For instance, oh, an online photo editor used by the user in isolation, on either local files or easily downloadable files. User doesn't like that instance, so they find the AGPLed source code and run their own, bringing all their files.

(Why, in that situation, would the user be entitled to the custom modifications in that instance they are abandoning? If you don't like that instance for whatever reason, but like the features, tough luck. Code your own.)