The reason cats purr is undetermined. Scientists know that a cat have 2 sets of vocal cords to produce different sounds. One set is for the meows (the lower set) and the other from growling and purring (the upper set) These sounds are made by vibrations from increased activity in the blood vessels around the vocal chords. Cats may purr when their are content but some also purr when they are in pain confusing scientist on the function of the purr. The purr can be loud and obnoxious or almost silent felt only by vibrations.