【Vertex and fragment programs】

　　When you use vertex and fragment programs (the so called "programmable pipeline"), most of the hardcoded functionality ("fixed function pipeline") in the graphics hardware is switched off. For example, using a vertex program turns off standard 3D transformations, lighting and texture coordinate generation completely. Similarly, using a fragment program replaces any texture combine modes that would be defined in SetTexture commands; thus SetTexture commands are not needed.

参考：file://localhost/Applications/Unity/Unity.app/Contents/Documentation/Documentation/Manual/ShaderTut2.html