This can be very useful if you are looking out for mail from a particular person or on a particular subject from a large amount of mail, or to find an email address if you have forgotten to put it in your address book. Alternatively you can drag the message to a deleted items folder, if you have one, or drop the mail in your wastebasket. When an email is deleted it usually is simply transferred onto a 'Deleted mail' folder, where you can still read it, and pull it back out into another folder if you change your mind, rather like a trash bin on your desk top. If you delete it from the delete folder, however, it is gone for good. It is a good idea to delete messages for good unless you feel you may need them again - they tend to mount up and take up space otherwise.
Mail is often pretty ephemeral.
Both the sender and whoever administers server space for you may not thank you for hoarding. The BBC is not responsible for the content of external internet sites.