He saved the country and struck a blow for democratic rule.
The idea of a Republic being able to endure dissension and survive it's tribulations and emerge as an attractive vehicle for government is directly attributable to the fortitude he showed as President. Lincoln understood what many at the time did not, that the eyes of the world were upon the United States. The destruction of the Union would have undermined Republican government everywhere and strengthened autocrats the world over. The government could not be dissolved due to the frivolities of election outrage, nor could it be torn asunder by violent rebellion. Especially not a rebellion led by aristocratic planters and slave holders.
He saved the country from becoming a splintered wreck. The secession of the Confederacy would surely have created an armed tension on the continent that may never have disappeared. Moreover it would have almost certainly prompted similar efforts both within the Confederacy and the Union has time wore on (some stresses in the Midwest, New England, California, Texas, and Georgia, could already be seen during the war). The result would have been a dysfunctional collection of polities unable to pool their resources, their genius, and their industry to become the greatest country on the face of the earth.
He prevented a slave holding aristocracy from arising as a major power in the Americas, and by his determination to prosecute the war as a liberating one struck a moral blow for millions of enslaved Americans.
Whats more he did all of this without destroying the Constitution. He had the foresight and wisdom to realize that it was better to bend and stretch that document than to allow the entire country to fall to tatters.
I have no sympathy for the rebellion, and I find this resurgent pro-South narrative that has cropped up within the past 3-4 years as childish.