lightning network – Purpose of LN transaction description

According to Bolt 11, an invoice may contain a short description. However, what I do not understand is who sends this information to whom.

Since the invoice is usually generated by the beneficiary, I would say that the brief description there is certain information for the payer, such as "Purchase: Product A, Product B". However, I've seen Lapps as mailboxes, it's the payer who creates the message for the recipient.

Or is it the idea that the beneficiary could offer a description, but the payor could then modify it as they please?