And the default, if someone doesn't choose any categories, would go to some proportion that Congress would decide. However, allowing even a "small" portion - which could still be billions of dollars - to be allocated directly by taxpayers would gather some very interesting data.
It would be very interesting to see which agencies would attract the most money, and what strategies they'd use to attract people to give their discretionary tax dollars to them.
Would people give more to the CDC, NIH, NASA, the Dept of Housing, Dept of Transportation, Dept of Homeland Security, ... ? We could start to get a direct temperature of what is ailing people the most and what they want their money to fund.
Government agencies could even "advertise," or at least try to get positive news articles that really highlight what they're accomplishing, to spread the word about the good they're doing. It could engage people more in exactly how their money is being spent, and care further about government, because they have at least a small hand in directly funding different departments.