The main purpose of this legislation is to change how married couples are treated under the tax code when it comes to state and local tax deductions. Specifically, it aims to eliminate what is known as the "marriage penalty" that affects these deductions. The goal is to make tax filing fairer for married couples by allowing them to benefit more from deductions compared to single filers. Key provisions of the legislation include adjustments to the income thresholds for state and local tax deductions.