Why was the balance between the number of slave states and free states so important in the pre-Civil War period?

The issue of slavery caused deep divisions, animosities and mutual claims since the early 19th century. The balance between slavery states and free states was fragile and had to be kept in order to maintain peace and the political unity of the Union. It was quite a precarious balance sometimes. It was held through legal, political and constitutional means such as the Missouri Compromise of 1820, the Kansas-Nebraska Act of 1854 and other pieces of hard negotiated legislation pieces.