Even when you receive a registered letter by classic post means, you have to identify and sign to receive your letter. This means identification is needed as well to receive a digital registered letter. The eID validation makes that a digital registered letter is also legally valid.
Furthermore, we ask the sender of a registered letter to validate himself by eID. After all, a verified user is a reliable sender.
This verification is only needed once, after which the user can smoothly receive all documents that need to be signed, like tenders and registered letters.