One especially useful thing about the spending site is that it explicitly lists the transfers from the federal government to the states. About 10% of the federal budget is transferred to the states, most of it for healthcare.
A useful thing about the taxation site is that it makes it readily clear how little of the government is financed from corporate taxes. Many people seem to think that if we only taxed them a bit more we’d be OK. Try this for an experiment: ask them by what percentage we should increase corporate income tax revenue. The idea that we’d need to increase it by 400% to get rid of the personal income tax isn’t on most folks radar screens.