The big difference between IMAP and POP3 comes into play if you use more
than one computer to access your email.
With POP3, if you read an email in one computer it will be removed from the server and won't be available if you try to download email with another computer.
With IMAP, the email messages remain on the email server and can be accessed with any number of computers or mobile devices. Changes made to your email on one device reflect also on all other devices connected to the same email account. For example, if a message is marked as "read" on one device, it will also show as "read" on subsequent devices as well.
In General, IMAP is the preferred email system for most clients as it enables them to access their email using more than one computer or mobile device.