The US's system of tax deductions makes it impossible for your employer to know how much to remove from your paycheck except in the simplest of scenarios. They may not know how many children you have or how much money your spouse makes if you file jointly or how much you pay in rent or whether you're a first-time home buyer or what your deductible medical expenses were for the year or whether you had education expenses and so on...
There is literally a form you fill out for your employer to tell them this information so they how to much withhold from your paycheck: https://www.irs.gov/pub/irs-pdf/fw4.pdf
A W-4 is an input into a formula that tells the employer how much you want deducted. So a couple with 3 dependent children and a mortgage will put down more deductions than a childless couple with no mortgage. But it's just to get you in the ballpark so you don't seriously overpay or underpay.
