The value methods are usually very expensive, because they create new Wikibase-DataModel-JavaScript objects from what's currently in the edit window(s) on screen. This is usually done for two purposes:

1. isInitialValue checks if the user changed something and the save button should be enabled. This is T124786: [Task] Widgets should delegate isInitialValue calls as far as possible.
2. stopEditing stores the changed value.

Neither of this requires the value method, when done properly. When all such usages of a value method are removed, the method should be removed.