Body piercings, scarification, and neck elongation all fall under the umbrella of tribal body modification. Plenty of body modification is so ordinary that most people don't even think about it. When was the last time you really thought through the implications of piercing your ears, after all? On the other side of the spectrum, though, are more extreme modifications, of which teeth-sharpening is just one notable example.
The practice has occurred within tribal cultures across the globe, and for a variety of reasons. Sometimes, folks without any particular cultural affinity take it upon themselves to sharpen some of their teeth as a form of personal expression not unlike a tattoo or piercing. Whether the reasoning is based in self-expression or shared values, the method is generally the same: someone's teeth are chiseled into the desired shape, turning the front incisors into something more fang-like.
More often than not, sharpened teeth don't arise from a practical need relating to mastication. The motives and applications can vary wildly, in fact, but spiritual rationales are common. In some cultures, sharpened teeth are seen as ways of allowing a person's soul to escape their body when they die. In others, they believe sharpening a person's teeth has greater aesthetic value, making the bearer of sharpened teeth more beautiful.
In other societies, a sharp tooth or two indicate status within the social hierarchy. Among Mayans, to cite one example, sharper teeth or even teeth inscribed with surprisingly elaborate designs were markers of high status. Attitudes about gender have also been expressed through this kind of tooth modification.
The practice, though rare today, still happens on occasion. The fact it persists highlights the importance of teeth sharpening as a custom for many different people.