Faces. We all have them (hopefully) and well all instinctively know how to read them them. A face, associated with a mascot or a spokesperson, has always been a powerful way to brand a company or a product. Apple has been using the Happy Mac Face, as well as the Sad Mac Face, for years.
Why? Because humans know how to read faces. Faces can convey a huge amount of information about personality, intent, mind-set, attitude, status, emotional state and most of all, uniqueness.
It should be no surprise that faces are used by many designers for their app icons.