The user wants to have full control over their data. Even after they have submitted it to the system, they may want to delete it.
The system tells the user how they can delete their data as well as the consequences of doing so, and provides a clear means to proceed with the action.
Allowing the user to delete their data at any time is an effective way to ensure the user is comfortable providing such data in the first place. Of course, the onus is on the system and organization to act responsibly and genuinely remove the data in question from storage, rather than just hide it in the UI layer.