It is done for various purposes, typically either for preservation, recreation, or performance. Groups that dance for different purposes tend to do dances differently, and to select different dances. Recreational dancers prefer dances of limited complexity, and tend to alter the dances (often unintentionally) according to their own tastes. Performers typically prefer dramatic, flamboyant, or athletic dances, and they often tend to dance with stylized techniques and exaggerated movements.
History
Traditional dances such as branles, the polka, et al., have been done internationally for hundreds of years. International folk dance has at least two more specific distinguishing features. First, the dances are typically considered to be primarily the products of national traditions rather than the property of an international tradition. Second, dances other than folk dances tend for the most part to be excluded.
The creation of international folk dance as such is often attributed to Vytautas Beliajus, a Lithuanian-American who studied, taught, and performed dances from various ethnic traditions in the 1930s. Other prominent teachers and promoters of international folk dance in its first few decades included Michael and Mary Ann Herman, Jane Farwell, and Dick Crum.
External link: http://www.phantomranch.net/folkdanc/folkdanc.htm