POP and IMAP, and practicality of Thunderbird

IMAP can be better if you travel a lot, use different computers to access your email, or sometimes have to access your email via a webmail interface (which may or may not be possible with your email provider).

Because IMAP leaves email on the mail server, even after you've read it (but not deleted it), you can still read it from other computers or a possible webmail interface. If you use POP3, it gets downloaded to whatever computer you use and deleted from the mail server, leaving you no way of reading the emails from anywhere else other than that particular computer.

Of course if you don't travel, or always have your computer with you then I guess this particular "advantage" of IMAP over POP3 is moot. Another reason some people prefer IMAP over POP3 is because of backup. If all your email is left on the mail server, typically it will get backed up by the email provider (although you should check with them that this is true). This means that you don't have to worry (as much) about backing up important email. Of course, as with any electronic data it's always wise to keep multiple backups, and not rely on any one particular service/media to keep you safe.

An advantage of POP3 over IMAP is that because email gets downloaded to your computer directly and then removed from the mail server, it no longer takes up space on the mail service. Most email providers have some kind of quota and depending on your volume of emails received, you can quite rapidly fill up your quota. Using POP3 eliminates/alleviates this problem.

Just my two cents worth.