The simple answer "Because I can" is probably premature at this stage. The main reasons I want to do it are principle and security.
As Paul points out in his page on encrytping e-mail [[How to Encrypt emails in Debian Etch 64 bit, PCLinucOS and Ubuntu Hardy Heron.]] plain e-mail is inherently insecure. It is sent in plain text through servers you do not control including your ISP's. However even if you encrypt your e-mails, as Paul suggests, an attacker who can monitor your internet connection can learn a great deal about you by studying who you send e-mails to and who you receive e-mails from. In security jargon this is called "Traffic Analysis".
Unfortunately an attacker in this context can mean your own government ISP's are being required by law to keep records of who is sending or receiving your e-mails for I think two years. The way your ISP does this is by keeping the logs of its mail servers. The way things are going I don't think it will be long before ISP's will be required to store the contents of your e-mails too. If you run your own e-mail server monitoring your e-mails becomes harder.Another reason to do this is to learn more about e-mail and the way it is distributed under Linux.