Complex sentence: The purpose of using two arguments instead of one, is to gather information of the signs of the inputs in order to return the appropriate quadrant to the computed angle, which is not possible for the single-argument arctangent function.
Split into simpler sentences: The purpose of using two arguments instead of one, is to gather information of the signs of the inputs in order to return the appropriate quadrant of the computed angle. This is not possible for the single-argument arctangent function.