Sometimes you just have a single public IP address (unfortunately ipv6 is not that widespread yet), and you still want to publish stuff like Outlook Web Access and other applications to the net in a secure way. If you want that, the easiest way to do so, is to just pass port 443 to the Exchange server. But this means that if you have other web apps, you have to run them on the Exchange server as well. And besides, not everybody wants to put an IIS machine directly out on the net..
One way to solve that, is by putting a reverse proxy like Apache or Squid in front of it. Read more ..