Shading question

I have met the following code somewhere in a directx book. /* This example uses a dot product to square the vertex diffuse color components. */ // The shader is shown below. ps.1.0 // Version instruction. tex t0 // Declare texture. dp3 r0, v0, v0 // Dot product squares the vertex color values, // color(v0) * color(v0). // Bright colors max out at white. // Dimmer colors yield gray. in this code, the square of the vertex color values are achieved by dot product. Isn''t that tedious comparing to just go v0 * v0; is there any special purpose of doing in that way?