Example: Person earns $24,000 gross income a year. The person is paid monthly $2,000 gross. The federal tax taken out monthly can vary depending on marital status and deductions. Let's say the person is single and has 0 deductions. That's means they are going to take out the most amount of tax. (they meaning your employer who processes your payroll and taxes) As an example, your federal income tax is $500 a month. The net income for this person is $1,500 a month and that doesn't include state tax. So for that year, the person would have paid $6,000 a year in federal tax. So, the next year, this person files their taxes and finds out the tax they owe is $4,000 so they'll get a $2,000 income tax return. In this example, the federal government got a $2,000 interest-free loan.
Example 2: same person earns $24,000 gross income a year. The person is paid monthly $2,000 gross. Let's say the person is really single and has no kids but puts down married and 10 deductions. That means they are going to take out the least amount of tax. Let's say that is $50 a month. So for that year, the person would have paid $600 a year in federal tax. So, the next year, this person files their taxes and finds out the tax is $4,000 so they'll owe $3,400. In this case, the government did not have an interest-free loan.
So for those who can't deal with saving money but do not want to give an interest free loan, they would adjust their marital status and deductions so that they would neither owe money or get money. In this example, person 1 would want to get as close as possible to getting $4,000 a year taken out, or $333.33 a month.