Controlling a remote computer can be useful for troubleshooting or for demonstration purposes. There are many expensive ways to do this and there are few free systems, such as VNC.
I've always liked VNC for remote control but it was not easy to use, especially when used with SSH encryption. ShowMyPC.com eliminates all that complexity and adds the additional service of allowing multiple units behind a firewall to communicate. This is not an easy thing because home users have rotating IP addresses. ShowMyPC can also be used for broadcasting a desktop for demonstration purposes. This often requires a lot of money. The expensive alternatives include neat features, such as voice, but it's been my experience that these advanced features don't always work well on less than great bandwidth.