It is part of the MIME email format. If you view an email message as “raw text” you will be able to see these internal codes.
Another thing to keep in mind is that those email standards so many years ago were theoretical. Only a few companies, one of them being Apple, ever bothered to implement them. Others, such as Microsoft and Netscape, came up with new, de-facto standards. Since they controlled the market back then, their interpretation ruled. Although these de-facto standards are still based on MIME, they greatly restricted what could be done with e-mail.
So here is “beleaguered Apple”, circa 2000, struggling to find a CEO and make it through the next fiscal quarter. They have a nice e-mail client. But if they let people pick and choose how attachments are going to be displayed, their e-mails are always going to be wrong on Outlook, and wrong in different ways depending on the version of Outlook that the recipient uses. Back then, it was trivially easy to compose an e-mail message, using Outlook, that Outlook couldn’t properly interpret. Back then, it was much the same with Microsoft as it is with Apple today. It was Microsoft that decided how email worked. So Apple found that hard-coding “inline” was most reliable, even if it annoyed some people.