[MarkSweep]

If you only have income from one W-2, two if you are married, yes it is easy. But if you have any variability in your income, it's hard to predict how much you tax you are going to owe. Not every income supports withholding (i.e. savings accounts) and the way you can set withholding varies by income source (W-2 supports a dollar amount per paycheck, RSUs often use a percentage). So getting the right combination of withholding settings to cover your tax liability across all income sources can be tricky.

[akerl_]

The overwhelming majority of US taxpayers do not have RSUs or enough savings for a 1099-INT to impact their taxes.