Think of an xy plane, if you have a vector that goes from 0 to 1,1 then the magnitude will be the sqrt(2) and the angle will be 45 degrees. atan2 takes the x and the y and returns the 45 degrees. It needs x and y so it will be correct for the whole circle. if you pass it just the ratio of x and y, it cannot determine the quadrant. In this case, if it did not take x and y into account it could be 1,1 or -1,-1.