Well, it ignores just the simple cost of day-to-day living. If your income has gone to zero, the bit of extra money you might get through the child tax benefit isn't going to come anywhere close to covering what it takes to feed a child, pay your rent and pay your utilities. We need to have some basic income.
Also, the child tax benefit and those programs are dependent on previous year's income, which might have been turned off now, and it's not going to apply now if there has been a current change of income. It's reliant on the previous year, which might have been a stable year. There's a lot of fluctuation in families in terms of their financial positions.