The difference between an Android emulator and a physical Android device without GPU

Question

I need to use the CPU to simulate the GPU, implementing the interface call of OpenGL ES.
I learned that both Mesa3D and SwiftShader can be implemented. However, I found out that SwiftShader can be used in the Android emulator, but it does not work on the physical android device without GPU. Why is it possible to choose GPU software simulation (Mesa, SwiftShader) on the Android emulator, but not on the physical android device without GPU?
So, what is the difference between an Android emulator and a physical android device without GPU?
Can you give me some suggestion?