I never have really understood the purpose credits at all. Paying people money regardless if they work or not makes no sense. Make it a big deduction given only for those who actually work.
It's a behavior incentive for things that the government wants people to do. .
By example: We want people to have kids who survive, especially poor people. There's
no way to get people to perform cheap labor for the next generation unless poor people have kids that survive. The birth rate in the U.S. was dropping steadily for a long time prior to the child tax credit being introduced. In the 1950s, we had birth rates as high as 25+ per 1000 residents, and by 1997 that number was about 14.5. It's why you see the huge baby boomer retirement group, and the relatively small GenX workforce. The introduction of the child tax credit coincided with the first (albeit small) rise in birth rate in 8 years.
And then it dropped again because it's not really that much of an incentive to give people $1000 to have a baby. So, you know, less than 14 per 1000 now. Looking good, folks.