The robot needs power for all its parts. There are two major power systems that need to be considered: the power for all the digital parts, such as the Raspberry Pi and sensors, and then the power for the motors.
Motors need their own power system for a few reasons. First, they consume far more electrical power than most other components on the robot. They may require different voltages; it's not uncommon to have low voltage and high current capacity supplies for motors. The other reason that they need their own power system is that they can cause interference. They can pull enough power that other circuitry has brownouts and loses power long enough to get into an inconsistent or reset state, which would lead to SD card corruption on a Pi, and they can also introduce electrical noise to a power line as they are used, which could cause digital parts to misbehave...
