First let me say that you don't have to fill out this form.  The purpose of the form is that you can take as an itemized deduction the greater of your state income tax or sales taxes paid.  Sales taxes typically have both a state and a local component (if you google sales tax for the city you live in you'll find the combined sales tax rate).  Typically the state income tax deduction will be higher than your sales taxes paid unless you bought a car, boat or RV or you live in a state that doesn't have an income tax.  Hope that helps.