The Missouri Compromise in 1820 became necessary due to the growing divisions economically, socially and politically between the slave-dominated South and the increasingly industrial and abolitionist North. But it was mostly needed because the American border and population was moving west.
When territories like Missouri reached a population of 50,000, they could petition Congress for statehood, but admitting Missouri as a slave state would mean adding two pro-slavery Senators and one pro-slavery Representative to Congress. The North did not want to add to the political power of the South or of slaveholders. The Compromise did admit Missouri as a slave state, and Maine as a free state so the balance in Congress did not change.
It also set the northern border of slavery at 36 degrees 30 minutes North Latitude, thus establishing a guideline with which other new states above or below the line would be admitted.
All the compromise did was delay the inevitable split that was the Civil War. It did not allow for the eventual reality that new states would be added that straddled the line, or were too numerous south of it, meaning the disastrous Compromise of 1850 became necessary.
I like Missouri :)