The United States’ biggest program to support lower-income workers is the earned-income tax credit. It supplements salaries by as much as several thousand dollars — paid when those workers file their tax returns.

There are some good reasons the program works through the tax code, rather than as a more traditional welfare program. Families do not need to make some special application to receive the benefit at a social service office; rather, it is applied automatically, as they file taxes. There is no stigma to receiving it, because it is just one more aspect of a complicated tax code. And for families whose income rises and falls, the earned-income tax credit also automatically pays benefits in years with lower income and takes them away when earnings turn out to be higher.

“When done through the filing of tax returns, people don’t have to go to an office, take time off from work, that sort of thing to claim their benefits,” said Bob Greenstein, president of the Center on Budget and Policy Priorities. “If the country were to contemplate moving various social programs for people with low or moderate incomes out of the tax code, it would be really incumbent to create something with service that is pretty different than what exists today.”

But there are also disadvantages to making the tax code the vehicle for social assistance. Some 36 percent of I.R.S. audits in 2017 were of families claiming the earned-income tax credit, even though those are usually households with relatively low incomes. Scammy providers of tax preparation services encourage some tax filers to lie about their situation. Even people trying to file their taxes with full integrity may run afoul of complex rules about whether a child counts as a dependent, for example.

It all means that for lower-income people, doing taxes correctly can mean the difference between receiving thousands of dollars in cash, or not. And the I.R.S. is in the position of deciding whether a particular household is worthy, the type of role that typically falls to welfare case officers in other parts of the government.

Many of the other areas in which the tax code administers public policy have even weaker claims toward some ideal of efficiency.

Take the case of 529 accounts for college savings and 401(k) accounts for retirement savings. Investment gains are untaxed for 529 accounts, and 401(k) accounts allow people to place untaxed dollars into an account on which any gains remain untaxed until funds are withdrawn. The potential advantage is highest for families who both have the means to save and would otherwise face high tax rates because of their high income.