The purpose of the pinna is to collect sound. It does so by acting as a funnel, amplifying the sound and directing it to the ear canal. In mammals like cats and dogs, it is obviously highly directional, but even in humans, the pinna functions as a directional sound collector. The "sweet spot" for which the pinna best collects sound is called the acoustic axis or the acoustic fovea. In various species, the pinna can also signal mood and radiate heat.
The frequency at which the pinna becomes a directional sound collector can be calculated since it does so at frequencies such that the pinna is larger than the wavelength: