They ask for all income so a calculation can be made for taxable income, the amounts withheld, and the remaining to be done in ES payments.
https://www.irs.gov/businesses/small-businesses-self-employed/estimated-taxes
Form 1040 ES provides an accurate estimate of needed payments based on taxable income, amounts withheld, and what "shortage" there might be to be paid via ES payments
https://www.irs.gov/pub/irs-pdf/f1040es
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