While the practice of scalping was relatively commonplace (but not entirely universal) in the region that's now the United States, there isn't a singular reason for why people were doing it. For the Apalachee in northern Florida, scalps were necessary as proof-of-kills in order for warriors to ascend in rank. For one of the Dhegihan nations (either the Osage or the Omaha, I'm forgetting which right now and I'm away from my sources at the moment), one of the reasons scalps were taken was to appease the dead - the enemy scalp placed at a grave site signified that deceased's kin had sent another to accompany the deceased in the next world (its probably related to an older Mississippian custom of performing human sacrifices at funerals for prominent individuals, since the Dhegihan nations are the descendants of the Middle Mississippians). For the Haudenosaunee and the Wendat (The Iroquois and Huron Confederacies, respectively) a scalp was a surrogate for a captive, a token that justice had been dealt to the enemy - ideally, a live captive would have been taken instead, to be adopted or punished by the women of the community that had dispatched the warriors in the first place, but that isn't always viable.

Once European colonists establish themselves on the continent, scalping takes on a new meaning and becomes much more commonplace. For one, the introduction of firearms and other European weapons made fatalities more commonplace. But more importantly, Europeans put bounties on the scalps of nations with whom they were at war, a fact that both Native and colonial fighters took full advantage of for their own profit. The United States continued this practice, as seen in this rather infamous advertisement published during the Dakota War of 1862.