A few years ago, I met a beggar at a bus stop, who said that the store across the street had a sleeping bag for 50% off until the end of the week, and he was hoping that he would be able to buy it for the winter. I didn't have much money on me myself (basically just the bus fare), but I gave him what I had and walked home instead. To him, those five-ish euros meant not freezing in his sleep, while to me, they only meant a bit of unplanned exercise.
If you have nothing at all, every little bit of money gives you access to some tangible improvement of your situation. But if you have much less than nothing, and every cent of additional income just serves to lower a gigantic debt, there isn't really much incentive to earn more when it makes no significant difference in the end. This is probably one of the reasons bankruptcy law allows discharging unpayable debts, because otherwise debtors might not be motivated to do something about the debts they could repay.
At the low end, at some point you can afford to have a roof over your head, and an extra $20 might mean that you're eating much better for a whole week, or you can buy new shoes, or whatever. But if you're homeless, $20 won't feed you for very long or get you out of the rain for very long.
At the mid range, a salary bump from $30k to $50k might make a world of difference if you're trying to raise a family. It might mean that you're not worrying about how you're going to pay for groceries, or it might mean that your kids are always wearing reasonable, well-fitting clothing.