The credit is calculated by taking 15% of your earned income above $2,500.
so it is possible that your 2019 income and your 2020 income are high enough that it has no impact on the child care credit.
But let say your 2019 income was $10,000 and your 2020 income was $5000, there would be a benefit to use 2019 - because the credit would be higher
