The wedge is used for parting or separating materials, and its first use by early humans might have been in skinning animals--separating a hide from flesh. Its unique function is transforming the power applied to its rear into equal forces pushing outwards from its surfaces.
In nature, wedges abound--anything that can pierce is a wedge, and there are many, many examples. One of the most vivid is the penis of the male bed bug.
During copulation, the male stabs the female's abdomen with his sharp phallus, bypassing her reproductive tract and injecting sperm directly into her body. Scientists call this "traumatic insemination," and though practiced with apparent evolutionary success by several species of invertebrates, it's not at all a positive experience for the female: the wound made by the male's wedge is liable to get infected.