The source of power in our homes, i.e. the AC outlet, is optimized for transmission. For many uses we have to transform that to what we need using a power supply. This video is pedantic at times in how high level it is but still a good introduction to what a power supply is:
It is HTML 5 and watchable at 1.5X speed.
It is HTML 5 and watchable at 1.5X speed.