Why did Germans fight in American Revolutionary War?

The British lacked sufficient troops to fight in all theaters of operations during the Revolutionary War (1775–1783). There was no draft at the time, so raising troops was often problematic. Therefore, mercenaries were commonly used by warring nations. Britain at first tried to recruit Russians. When this failed, Germans were recruited.
German troops employed by the British were known as Hessians. This is because most came from Hesse-Cassel. The term is misleading, however, because a number of other German states also provided troops. Some 30,000 German troops fought under the British flag, and they represented more than 30% of all British forces in North America. The quality of the troops was uneven, and some of them gained a bad reputation for looting.
In December 1776, Washington won a key engagement against the Hessians at the Battle of Trenton. The Hessians had not fortified their camp, so Washington won a smashing victory. Nine hundred Hessians were captured, and Washington's victory was vital for the Americans's flagging morale.
Some Germans also fought for the Americans. The most notable among these men was Baron von Steuben (1730–1794). Steuben had served in the Prussian army and had been on Frederick the Great's staff. He was put in charge of drilling the Continental Army. His drills helped turn Washington's army into a disciplined fighting force.