Why was it so important to the south admit missouri as a slave state?

Missouri was important to the South that it be admitted to the Union as a slave state because it could tip the balance between pro-slavery and anti-slavery states in the Union.  This had become a very sensitive issue.  To solve this dilemma, it came up with the Missouri Compromise of 1820.  Under this compromise, Congress was allowed to admit Missouri as a state but it also admitted the Main to the Union as well.  This kept the balance between the states and maintained the peace.