They are set up in brackets to directly benefit the poor. Higher wages pay in higher brackets, lower wages in lower brackets. On top of that the money goes towards an institution devoted to giving them further low bracket groups further refunds and credits to get that little amount they pay back in their pockets.
The poor essentially net no tax income to the state whereas the higher income brackets are left holding the bill.
The government needs to have revenue somehow. Some people think a sales tax would be better. BUT a sales tax disproportionately effects the poor. It would also discourage consumers from buying things and so decrease revenue in businesses and so they would have less money to pay workers.
An income tax is the best way.
If you are a person in America and you work hard, you can make money. This is the core idea of the American Dream. If you are a business owner, and you want to your business to do better, you work hard. If you work hard, you get rewarded with more money. This is the core idea behind capitalism. If you employ 10 workers in your small business, then when your company does well, those 10 workers do well also. If you are able to expand your business then you hire more workers for your business. All this improves life for your workers, the so called lower class. Now we are going to introduce income tax into this situation. Income tax increases when you make more income. So, when your business does well, you lose more money to taxes. This in turn reduces the money you can invest back into your business. This kills growth in your business. That kills the improvements your business is making in the lives of your employees. That's how income tax is not beneficial to the poor.
Now some may argue that the money the government is taking will be returned to the poor through welfare. This may be true, but even if all income tax money goes into welfare, not taking the money in the first place would be better than taking it. This is because the government is taking some of that tax money and using it to pay people to move the money from the business owner to the lower class workers.
Income taxes are for those who are working in a job, earning money. The less money you have, the more you need. Therefore, it would be inadvisable to pay as high income taxes as it is. Either a flat tax or percentage would be a great way of having taxes paid, but that's just me rambling.