The purpose of the wigs was simply for fashion, the purpose for the powder was keep them clean and parasite free.

because it was thought to be fashionable and they powerd them because of head lice

Apparently, it was Louis XIII of France who started the fashion. He went bald and covered up the fact with a wig; the aristocrats in his court emulated him and , as they say, the rest is fashion history! http://www.costumes.org/history/100pages/18THHAIR.HTM

In 1624 Louis XIII went prematurely bald. He disguised this with a wig and started a fashion which became almost universal for European upper & middle class men by the beginning of the 18th Century during his similarly follicley challenged son's reign. Around 1715, lighter colored wigs were in fashion so, after unsuccessful attempts at making the color of bleached wigs stable, people started to use powder instead. Hair powder was made from finely ground starch, scented with orange flower, lavender, or orris root, and occasionally colored blue, violet, pink or yellow, but most often white.Powder rapidly became an essential for full dress occasions and it continued in use until almost the end of the century. 17th Century Charles II (of England, another notable style leader) also showed signs of male pattern baldness in middle age, and switched to wigs. Other reasons included ease of hairdressing (send your hair out to be done, you don't have to sit for hours in curlers), ease of cleaning (if you got lice you could boil your wig and shave your head and ----zip-no lice), comfort while sleeping (short hair beneath), ability to change styles/color as easily as putting on a hat, and class considerations (wigs were expensive and looked it). After 1790, both wigs and powder were reserved for older more conservative men, and ladies being presented at court. In 1795, the English government put a tax of hair powder of one guinea per year which effectively caused the demise of both the fashion for wigs and powder by 1800. In France the association of wigs with the aristocracy caused the fashion for both to evaporate during the terror of 1793. After the 1720’s, shorter wigs were more popular. Wigs were made of horsehair, yak hair and human hair, the latter being the most expensive. http://www.costumes.org/history/100pages/18THHAIR.HTM Side Note: Wigs have been worn for thousands of years; the ancient Egyptians, for instance, wore them to protect their shaved heads from the sun. Other ancient peoples, including the Assyrians, Phoenicians, Greeks and Romans also used wigs. Curiously, they are principally a Western form of dress; in the Far East they have rarely been used except in the traditional theatre of China and Japan. After the fall of the Roman Empire the use of wigs fell into abeyance in the West for a thousand years, until revived in the 16th century as a means of compensating for hair loss or improving one's personal appearance. They also served a practical purpose; the unhygenic conditions of the time meant that hair attracted head lice, a problem that could be much reduced if natural hair was shaved and replaced with a more easily de-loused artificial hairpiece.

beacuse they would just shave there head and wear a wig and since they couldnt shower everyday they would shower once a month. they would put there wig like in somthing like a stove so it kills all the bugs they prevented lice also...