We already knew AIs can reasonably approximate high or drunk status at will, the ability to be drowsy isnt exactly out there. Slow processing speed, put little skips in input so they miss things going on around them, and bobs your uncle. As for why, the same reason why for literally anything humanish they do. Because. I did like the idea of her being an experiment in a chassis thats as close to being alive in the biological sense as possible. Not just for the greater ability to do human stuff, but even self healing capabilities would be nice, albeit slower than going to a mechanic. Or operating as a bridge between man and machine.