Sine/Cosine encoder: Fine position interpolation
Consider this:

Now assume you have a sin value of 0.0.  Where are you?  x = 0 or x = pi?  You cannot tell from this information.  If, however, you also have the cos, then you know from it that you are at x = 0 if cos(x) = 1 or you are at x = pi if cos(x) = -1.