How to declare a fragment that has no interpolation (TEXCOORD0,1,2)

Actually, I don't understand why the author wrote the pixel shader that way, because if I don't interpolate the sun color, why do I need to pass the sun color per fragment, we can pass it per frame or per object? right?

[code]

struct PS_INPUT

{

….

NOINTERPOLATION sun_color : TEXCOORD1;
};

[/code]