*** Improved by Wrobinson on 2/1/2008 ***
Front-end servers in Exchange 2003 and 2000 are used to add a layer of protection to the production Exchange environment; however, mail hygeine should be performed on the back-end as well. The reduction in load is due to the offloading of client connections typically from the Internet (Outlook Web Access, RPC/HTTPS, IMAP and POP for example) to the front-end servers instead of connecting directly to the back-end servers.
It is also possible for a front-end server to reside securely on a corporate network using a reverse-proxy such as ISA.
For more information on front-end and back-end back-end topologies for Exchange 2003 and 2000, review the following TechNet reference http://technet.microsoft.com/en-us/library/aa996980(EXCHG.65).aspx
*** End Update ***
Front end servers protect your production exchange server and reduce the load by allowing you to run anti-virus and spam scanning before the mail hits the main production server(s).