They go into that in the RFC. Basically, the idea was
1. Mail is delivered to the server.
2. You check your mail. All your mail is downloaded to your client
3. Once the download completes, the mail is immediately deleted. The only copy left is on your computer.
The original POP standard is
RFC-918. See "The Normal Scenario," a couple of pages in.
In this early version, there is no delete operation. Deleting is part of receiving! Support for receiving without deleting was only added later.
This worked fine when most people only had one computer. You'd go into work, download all of your email onto your (one and only) computer throughout the day, deleting it from the server as you went along. You didn't expect to get your email on your home computer, your laptop at Starbucks, your work computer, and your cell phone. Note the date on that RFC: 1984.
There's a reason they called it the "Post Office Protocol." The model was a rural post office, where people would come in once a day, shoot the breeze with the mailperson, collect their mail, and take it all home with them. (I know people who actually still do this.)
If you read through the later versions, (The latest is the shiny, new
RFC-1939, from 1996) people had deviated from the old model and were configuring their clients to
not immediately delete mail; as mentioned above, it suggests that such situations are better suited to IMAP.
...users and vendors of POP3 clients have discovered that the combination of using the UIDL command and not issuing the DELE command can provide a weak version of the "maildrop as semi-permanent repository" functionality normally associated with IMAP....
By now, people have pretty much exclusively migrated to the "maildrop as a
permanent repository" model, so much so that newcomers think that deleting old mail from the server is "crazy."