Why was it important to keep the balance between slave and free states

The continuation of the balance between slave and free states was important given the nature of the Congress and the balance of power between the competing sides in the issue. ... Compromises were needed to equalize the power between proslavery and antislavery interests in the government to keep the Union together.

the name of the theory is landmass theory.

1.   île de la cité

2. 200 years

3. for the nave and choir

hope im right, havent learned about this in a year