Quarterly estimated taxes are how many self-employed people prepay tax during the year instead of waiting for a large surprise at filing time.
Why they exist
When no employer is withholding tax for you, the IRS still expects tax payments to arrive during the year. That is where estimated payments come in.
What people usually set aside
Many freelancers use a percentage-based reserve system, moving part of every client payment into a separate tax savings account so quarterly due dates do not feel catastrophic.
Bottom line
Quarterly taxes are easier when they are treated as a cash-flow system, not a last-minute panic problem.