Form 1040 - ES payment voucher for 2021: why would I pay estimated federal tax early? Wouldn't I wait till December to pay and earn income on the cash in the mean time?

[VolvoGirl]

Because you are suppose to pay your tax evenly during the year or as you earn it.  If you don't then you might end up owing too much and incurring a penalty.   It's up to you.  The estimates are optional to pay.  They don't get sent to the IRS with your return so they won't be expecting them.

[fanfare]

Refer to IRS Form 2210 for the payment due dates and the penalty for not complying with those dates.

[VolvoGirl]

Yes see, Form 2210 https://www.irs.gov/pub/irs-pdf/f2210.pdf

And 2210 Instructions  https://www.irs.gov/pub/irs-pdf/i2210.pdf