Good question. When you snap MOSS modules together, you're not only building the physical body of the robot, but you're also building its behavior. Connecting sensors to actuators in novel ways will equip your robot with interesting reactive behaviors that can appear surprisingly intentional.
If you really want to write computer programs for your MOSS creation, you'll be able to do that too. Details on programming here: http://www.modrobotics.com/blog/?p=1503