Why do you recommend "server mail" what's the benefit?:

Microsoft Exchange (also included in the Office 365 subscription product) is a "Server based mail product" all the mail is maintained and stored on the server.

When using server based mail you start up Outlook, it makes connection to the servers and then synchronizes mail, calendar events, contacts and tasks both ways up and down (from and to your PC):

  1. Changes (new email, moved mail, created folders mark as read etc.) that you may have made in Outlook when you were offline (such as on a laptop or if the server was down) will be sent back up to the server
  2. Changes (new email, moved mail, created folders mark as read etc.) that we made on another computer or via web mail or a smartphone will be synchronized from the server down to your Outlook.

Additionally, Exchange server provides the same premium experience on mobile devices and tables such as Windows Phone and tablets, Android, and IOS.

The huge advantage to this is that all your computers/devices are kept in sync and changes made in one place (such as read and delete actions) will be mirrored to all your devices/access methods.

Isn't this the same as the free IMAP service I can get from my ISP?

If you have experienced IMAP you will have some understanding of how this works but IMAP is extremely limited by comparison. There are a number of other features such as shared folders calendar collaboration, remote wipe for your phone (for lost phones) available in exchange but that's enough information for now.