Since POP3 creates local copies of emails and deletes the originals from the server, the emails are tied to that specific Desktop machine, and cannot be accessed via any webmail or any separate client on other computers. At least, not without doing a lot of email forwarding or porting around mailbox files.
Compared to POP3, IMAP allows users to log into many different email clients or webmail interfaces and view the same emails, because the emails are kept on remote email servers until the user deletes them. In a world where we now check our email on web interfaces, email clients, and on mobile phones, IMAP has become extremely popular.
It isn’t without its problems, though.
Because IMAP stores emails on a remote mail server, you’ll have a limited mailbox size depending on the settings provided by the email service. If you have huge numbers of emails you want to keep, you could run into problems sending and receiving mail when your box is full. Some users sidestep this problem by making local archived copies of emails using their email client, and then deleting them from the remote server.