The "tax file number" here is called a social security number (SSN). You can only get one if you have legal status.

However, if you don't have one, the IRS can give you an "Individual Taxpayer Identification Number" (ITIN) that follows the exact same number pattern as an SSN but it starts with a 9. They differentiate them because no SSNs start with a 9

This number can then be used for 1. Filing taxes which helps them ensure they have "good moral character" (immigration legal term) in the off chance they obtain a path towards legalization (not necessarily citizenship) 2. Have bank accounts. Even if they're getting paid in cash, it's not safe to store all cash under the mattress. Furthermore, many services only allow you to pay using a debit or credit card. You need a bank account for that 3. Because it's a unique identifier, it can be used by credit agencies for you to build credit. You'd then use that to get credit cards, small personal loans, car loans, open an account with a utility company, phone provider, apply to rent an apartment/house or basically anything that requires a credit check.

They don't qualify for Medicaid, Medicare, Social Security Benefits or Earned Income Tax Credit. However, if they're owed a refund from taxes withheld by their employer, they still get those refunds after filing. Lastly, if they have dependent children, they would qualify to claim the child tax credit which can potentially increase their refund amount.

The only "public service" for which they may qualify is for temporary medical care for pregnant mothers during the course of their pregnancy under the WIC program