Yes, you really need to fill it out. It is required by law for Form I-9 to be completed and retained for every person hired after November 6, 1986, in the U.S. as long as they work for wages or payment.
The only exceptions are; individuals hired on or before November 6, 1986, individuals hired for casual domestic work in a private home on an irregular basis, independent contractors providing labor to you if they are employed by a contractor providing contract services (think staffing or temp agencies), and individuals not physically working in the U.S.
Its importance is based in The Immigration Reform and Control Act of 1986 (IRCA), the Immigration Act of 1990, and the Illegal Immigration Reform and Immigrant Responsibility Act of 1996 (IIRIRA). These Federal Laws prohibit employers from knowingly hiring unauthorized aliens, prohibits discriminatory documentary practices during the employment eligibility verification process and the IIRIRA created/updated the list of documents that can be used on the I-9 to verify eligibility status. The Form I-9 is now maintained by the Department of Homeland Security, U.S. Citizenship and Immigration Services.
You should have and retain a new hire packet of required paperwork from both the Federal and State level for all new employees.