Ask any woman, they wear thongs to prevent visible panty lines when wearing tight and stretchy dresses or pants because there is no material on the back with thick seam or hem to create an outline. In recent years there are seamless panties made from very thin and smooth material and no hemming, but they are still somewhat visible unless they are extremely well-fitted.

Many primitive or ancient societies have loincloth for men, including the Aztecs, Aboriginals, Bushmen, and Japanese fundoshi. In the 1920's and 30's many exotic/burlesque dancers wore something similar to thongs or g-strings. For example Josephine baker wore a string of bananas and a thong for many of her performances. (For more info read Striptease: The Untold History of the Girlie Show.)

According to A Century of Austrian Design: 1900-2005 by Tulga Beyerle and Karin Hirschberge, Austrian designer Rudi Gernreich first made a range of swimsuits and underwear featuring thongs (In addition, the book mentioned Gernreich was born in Austria, where it was common to exercise in the nude or in minimal clothing such as a thong in the 20's. Hitler banned nudity after the annexation of Austria. Gernreich created many minimalist clothing in addition to the thong, including very thin and wirefree "No Bra" and monokini.)