The US federal government is far and away the number one source for student financial aid. More than 2/3 of all US college students receive one or more forms of financial aid from the US Department of Education, and that aid adds up to tens of billions of dollars every year.
The ONLY way to apply for federal student aid is by completing the Free Application for Federal Student Aid, better known as the FAFSA.
Because the FAFSA form is so ubiquitous, most states and colleges rely on that exact same information to determine who is eligible for state aid and institutional aid. Some schools ask for additional information - many of these schools use the CSS-Financial Aid PROFILE application, in addition to the FAFSA form.
Though technically you might not need to complete the FAFSA to apply for private educational loans, the financial aid office is required to certify your financial need to all educational lenders, so it's highly likely that your school would require you to complete the FAFSA even if you were electing not to apply for federal, state and institutional financial aid.
There really aren't any good reasons not to apply for federal financial aid, as the federal forms of aid are the most numerous, the most available, and the most attractive forms of aid. The federal government's student loan programs, the Stafford and the Perkins Loans, are far cheaper and easier to get than any other educational loans. Stafford and Perkins loans have low, fixed interest rates, and do not require a credit check or a cosigner, while private educational loans are generally high-rate variable interest loans that are rarely available to student applicants without a cosigner.
Completing the FAFSA is not a requirement to admission for any US college or university, but you would be hard-pressed to any significant financial aid without taking that route.
If you need more information about the federal aid program, I highly recommend the US Department of Education booklet that I've linked below - it's called "Funding Education Beyond High School: The Guide to Federal Student Aid", and it explains the entire student aid system, including scholarships, private educational loans, and the entire federal aid program.