Because amplitude and phase data seem more intuitive, you might assume you should use polar amplitude and phase data instead of Cartesian I and Q data. However, practical hardware design concerns make I and Q data the better choice.
Precisely varying the phase of a high-frequency carrier sine wave in a hardware circuit according to an input message signal is difficult. A hardware signal modulator that manipulates the amplitude and phase of a carrier sine wave would therefore be expensive and difficult to design and build, and, as it turns out, not as flexible as a circuit that uses I and Q waveforms. To understand how to avoid manipulating the phase of an RF carrier directly, refer to the following I/Q modulation equations: