A POP3 access allows you to send and receive your email messages from a desktop email client like Microsoft Outlook, Thunderbird, Entourage, etc. A user need not login to the web interface and face hundreds of banner ads everyday, but just wait till the program fetches the mails from the background.
POP3 access (and even IMAP) has always been free when you’re using a GMail account. However, by default, for Yahoo! Mail, you will need to upgrade to at least the Yahoo! Mail Plus account (yes, you have to pay in this case) in order to avail of the POP3 access.