Why is the Missouri Compromise significant?

The Missouri Compromise was an attempt to head off the
slavery debate which was gradually heating up. The issue was not so much the existence
of slavery, as its extension into the West.  Stephen A. Douglas proposed the Compromise
as part of a plan to have the Eastern Terminus of the Transcontinental Railroad in
Chicago in his home state.

Missouri petitioned to come into
the Union as a slave holding state. This would have upset the balance of slave and free
states in the Union. The free states already had a majority in the House of
Representatives; but the Senate was evenly divided. Were the slave states to achieve a
majority; they might institutionalize slavery with a constitutional amendment. If the
free states were to gain a majority, they might attempt to eliminate slavery altogether,
again by Constitutional Amendment (both sides agreed that as onerous as slavery was, it
was constitutionally sanctioned.  The plan called for the admission of Missouri as a
slave state, and Maine as a free state; whereby the division in the Senate would be
preserved. It also provided that slavery would not exist above the parallel which
constituted the bottom of the Missouri state line.

The
compromise is important in that it delayed the slavery debate for a short time. It did
not end it. Later, the compromise was declared unconstitutional by Justice Roger Taney
in the Dred Scott decision, and the slavery debate heated up
anew.