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?

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.

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

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