i think a few other posters have hit the nail on the head:
the simple fact is that the average family (say 1 - 3 kids, mum and dad, one of whom works fulltime at least, possibly part time job for the other one) cannot afford to live on the average wage
there's no getting around that.
take my sister and her DH. he works more than full time (40 - 42hrs a week normally, although some weeks are worse than others). he's on just above minimum wage. my sister has a part time job for 12 hrs a week (3 days a week, mornings). mum looks after her 2 kids, so cheap childcare.
with BIL's wages + sis's wages, they couldn't afford to pay their bills without tax credits and top ups. it's literally a case of they couldn't pay their commitments, and they are NOT extravagant. secondhand shoes, no meals out, scrimping for school trips - i genuinely admire my sister's ability to cope how she does, they're not in any non-mortgage debt and managed to get a mortgage to be paying off (just).
they seem to be in a fortunate position, right? well when she's on the phone crying, asking me how to juggle the latest bill that's come in, and in the same call tells me about the damp problem that's getting worse in the kitchen with mold on the ceiling... well that is the face of modern britain. hard working couple, seemingly fortunate because they have bought property....
the simple fact is that your average family can't afford to live on the average wage!
the ONLY people who can, without tax top ups, are those in extremely cheap parts of the country re: house prices/rent and/or the childless, either by choice or not.
this scenario had a huge effect on us delaying having children - in the end we still couldn't really afford them, despite working in our roles for years.
the government talks about how women are delaying motherhood for years and years - is it any wonder? paying off student debt, paying for childcare, thinking about paying into a pension, looking after elderly parents whose pensions are now worthless - it's all these strains on your average family adult that means without these tax top ups the country would be chaos.
i've read that if you're on minimum wage with a couple of kids, the income is actually topped up to 20-22k? I think it was a link somewhere on MN.
so it's massively worth earning your first 10k of wages... worth earning up to 20k... but anything between 20k and 40k as combined family income, you feel the pinch so much that you barely feel the extra value from all that extra study or work. it's crazy.