WebMail provides a way of checking your email over the Web from anywhere in the world. You can use WebMail to simply check your e-mail while you are on the road, or as a complete replacement to your off-line email client!
WebMail uses the IMAP protocol, info on it can be found here. Email stored on an IMAP server can be manipulated from a desktop computer at home, a workstation at the office, and a notebook computer while traveling, without the need to transfer messages or files back and forth between these computers i.e. you do not have to physically download the email to your computer.
If you are using WebMail and retrieve email using your POP3 email program (e.g. MS Outlook, Eudora) those emails will no longer be available to WebMail as you will have downloaded and removed them from the server to your computer.
Any messages located within the WebMail Drafts, Sent or Trash folders will only be available through the WebMail interface i.e. they will not be available to your POP3 email program (e.g. MS Outlook, Eudora).
This facility is useful if you ever were at a friend's house, on a trip, at home or work and away from your computer and wanted to check your email (because it is a hassle to set your POP3 email up on another computer and then delete it again). WebMail is designed to be a supplement to your normal email program.