Meaning of Q and I in all-software QPSK communication systems?
A point in 2D space can be represented by either X and Y coordinates, or a distance and angle (relative to some reference point and axis).  However, the point measurement errors or noise is some common real systems is more likely to be uniform in the the units of an X,Y coordinate system, which may make error analysis and numerical error control in this space more tractable, thus lowering the computational requirement for most types of processing, even in purely digital systems.