As the debt limit climbs, the United States government has had to place funding in other places. Taxes only do so much, and are applied to the entirety of the government; spread out over every program and branch, each taxpayer can only do so much. While Americans can already give gifts to the treasury to help with the debt, the money is applied generally into the entire debt pool.
A more selective option would be to allow gifts to specific areas of the government, such as the Department of Defense or NASA. This would give certain branches of the government additional funds, enabling them to accomplish more without adding strain to the national debt. This would also show general public opinion based on donations.