1. Send out thousands of letters expecting some to be returned. They may be returned due to deliverability issues, or they may be returned with a reply attached or (probably less commonly) scrawled on the pages of the letter itself. Replies to letters are of course common whether they're expressly requested or not.
2. Give each letter a unique number in your database so you can cross reference the letter to the recipient information (including but not limited to the address) you have stored in your system. The letter may be returned with something else (e.g. another letter) attached so it's important to keep that information correlated.
3. Scanning the original letter is a low cost way to maintain this correlation. When the letters are returned you scan them then send them through a program you have set up to update the system accordingly. The program uses some primitive OCR and probably a checksum to automatically recognize the codes in the original letters. I can imagine this being used to automatically mark bad addresses if a letter is returned without additional context, but its main purpose is probably to route the letter - and any attachments, like other letters - to the appropriate agent.
To support a workflow not unlike the one described above, it is requested that the unique number that identifies the letter be left unobscured. This way OCR can do its job, deliverability issues can be flagged with minimal human involvement, and replies to letters can be put in front of the right person without creating too much organizational overhead.