What is the purpose of income tax deduction?

A deduction on your income tax return would reduce your taxable income on your 1040 income tax return and reduce your federal income tax liability.

An income tax deduction amount from your gross pay would be a prepayment of any future federal income liability you may have after your income tax return is completely at the end of the tax year and if enough is deducted from your gross pay you could end up receiving a refund of some of the withheld income tax amount.