The way you fill out a W-4 changed drastically for the 2018 tax season. There are no more personal exemptions. So that's why it's important to read the instructions that come with the W-4. If one doesn't read the instructions, chances are high they will over-claim on the W-4 and not have enough taxes withheld. Then they'll end up owing an under withholding penalty at tax filing time.
On the other side of that coin, if you under claim on the W-4 then you're basically allowing the Govt to use your money for the entire year, interest free. For some, the reduction in take-home pay matters more than gietting a big 4-figure tax refund.
