If you ask me for a contact email I will tell you something like [email protected]. Editing the FROM field allows me to write an email to you.

If you ever start sending me spam I can just block that address. Especially helpful with companies.

You can also use it for sorting emails.

It's honestly a basic feature of an email client. Even if you don't have your own domain stuff like the plus sign for aliases exists with many providers (or at least with gmail).