The best example is when using the laptop on battery, the integrated Intel GPU is used since it uses less power.

The purpose of having two GPUs is to extend battery life in laptops and to have better graphic performance for gaming, programming, and so forth.

The primary is the Intel integrated GPU (built in the processor) and the second is the discrete AMD or Nvidia GPU. Some Dell laptops have two Graphic Processing Units (GPU) built into the laptop. Integrated and Discrete Video General Information