In my opinion, this is one of the most ridiculous fashions I've ever seen. Ubangi women put plates in their lips because it is fashionable to have lips like a broad billed duck. I can't imagine how women feel having themselves go through this tradition!
According to Wikipedia, the custom is maintained today by a few groups in Africa and Amazonia. Some scholars have argued that in Africa, labret fashion boomed because slave traders refused to purchase slaves whose appearance was not considered "pristine" for the prospective buyers.

In Africa, a lower lip plate is usually combined with the excision of the two lower front teeth, sometimes all four. Among the Sara people and Lobi a plate is also inserted into the upper lip. Other tribes, such as the Makonde, used to wear a plate in the upper lip only. In many older sources it is reported that the plate's size is a sign of social or economical importance in some tribes.

In some Amazonian tribes, young men traditionally have their lips pierced when they enter the men's house and leave the world of women. Lip plates there have important associations with oratory and singing, and the largest plates are worn by the greatest orators and war-chiefs, like the well-known environmental campaigner Chief Raoni of the Kayapo tribe.

In South America, lip plates are nearly always made from light wood. In the Pacific Northwest of North America, among the Haida, Tsimshian, and Tlingit, lip plates used to symbolise social maturity by indicating a girl's eligibility to be a wife. The installation of a girl's first plate was celebrated with a sumptuous feast.