Like many of the parts of the human body, toenails do not serve a purpose, and are thus labeled vestigial structures. While toenails do provide some protection to soft tissues on the tips of the toes, they are considered to be leftover from the ongoing process of human evolution.
Vestigial structures are structures in an organism that have lost all or most of their original functions or purposes due to the process of evolution. Toenails are considered to be a vestigial version of fingernails. In primates, both fingernails and toenails are often used for scratching, digging or grabbing certain objects. In the process of evolution, humans lost their ability to use their toes as well as they use their fingers, as actions of digging and scratching were no longer as necessary to human beings.
Fingernails still serve a purpose in human bodies. They provide necessary protection to the tip of the finger bone, which is fragile. The bone in the toe, however, is much stronger than the one in the finger, and it doesn't need the protection of a nail in order to prevent damage. The tissue under the toenail is very sensitive, and in some ways the toenails are used to protect it. However, since the original purpose of the nail is lost, it is considered a vestigial structure.