As it was explained to me by a lawyer relative (for my jurisdiction, perhaps the theory is different elsewhere), it is to reimburse the custodial parent for expenses that would normally be shared if the parents were not separated. That is, they would be spending the same amount on clothing, food, lifestyle, etc. for the child either way, and are being reimbursed for the non-custodial parent's share of the expenses.

This is why the non-custodial parent cannot demand their child support money be used for certain things, or to give money directly to an older child (teenage or so) who could in theory manage their own funds. The money has already been spent by the custodial parent and they are simply being made whole with child support funds.

(I have heard of many cases where parents become unreasonably angry about not being able to dictate how child support money is spent, to the point of being censured by the judge. And similarly petty / spiteful behaviour e.g. parents quitting their jobs to try and reduce their child support obligation based on their income, and then promptly having the judge verbally tear strips off them - and not reduce the child support as it based off of their demonstrated, not current, earning capacity)

It doesn't really matter if both parties are obscenely rich and the child support will probably never matter in any meaningful sense. The custodial parent should still not be out-of-pocket due to the separation.

Family law is a wild place sometimes :)