Most of the time, the RCX will be on the top of your robot. You will have a tendency to pick the robot up by the RCX. Therefore it is important that the RCX is securely fastened to your robot's body. One way to do this is by using cross bracing. Here is an example of a poorly built robot. The robot's body falls off when it is picked up by the RCX.
Here, cross braces are used to solve this problem. By cross bracing the RCX to the robot's body, the RCX is much better secured to the robot, and it won't fall apart when picked up.
See also: Cross bracing, Lego cross bracing combinations