SSL enables browsers to communicate with a web-server over a secure channel that prevents eavesdropping, tampering and message forgery.  You should always use SSL for login pages where users are entering usernames/passwords, as well as for all other sensitive pages on sites (for example: account pages that show financial or personal information).

Configuring SSL on Windows with previous versions of IIS has been a pain.  Figuring out how to install and manage a certificate, and then associate it with a web-site, is something I bet most web developers don't know how to enable.