With the IMAP protocol your local email client is mirrored on the Server. If you delete a local email it's deleted on the server. This means all your email is available on every device you use. If you are constantly doing business out of the office - this works extremely well. However it does require you to have a high degree of organisational ability. If you don't manage your email well you end up running out of space on the server. For instance if you don't delete your trash your trash folder on the server still contains all that rubbish.
POP3 on the other hand is great if you mostly do work in the office. Once email is downloaded it's deleted from the Server - you never run out of space. You do want to make sure that you do backups of your email client.