most robots only have microcontrollers which are limited in terms of the size and complexity of the program you can store and have limited amounts of memory as well... PC's (and laptops) can do much more complex processing... Let's say that your robot uses "vision" as a type of sensor... you would need a PC to run the complex data analysis of the vision data (using software like RoboRealm for example) that the robot collects. Another example might be Artificial Intelligence... a microcontroller can't provide all of the processing that a good AI system would require.
