Why do I need to fill out an ESTA application?

The 9/11 Commission Implementation Act of 2007 (9/11 Act) amended Section 217 of the Immigration and Nationality Act (INA) to require the Department of Homeland Security (DHS) to implement electronic travel authorization and other measures to improve the security of the Visa Waiver Program. ESTA adds another layer of security that allows DHS to determine in advance of travel whether an individual is eligible to travel to the United States under the Visa Waiver Program and whether such travel poses a risk to law enforcement or security.