Because it was when America was separating from the British to make america its own country.
It should be to pay off the national debt
After the surrender of the Confederate Army, Lincoln is extremely conciliatory to the South. The reason why he does this is because he wants the Confederate States to rejoin the United States in a way that will help the country move forward peacefully.
With this in mind, Lincoln introduces the 10% plan. Only 10% of citizens within a particular state had to vote in agreement to join the United States again. The only condition of rejoining the US that the citizens had to agree to was the promise to never secede from the US again. This plan made it easy for the Southern states to rejoin.
