I think that the main impact of the abolitionist movement is that it helped to end slavery in the United States. Of course, it was not mainly the abolitionists who did this -- it was the Civil War. But the abolitionists helped to bring about the Civil War and helped in some ways to make the war be about slavery.
If it had not been for the abolitionists, there would have been less animosity between the North and the South. The two sides would still have had major differences, but they would have been somewhat less emotional. The abolitionists tried to demonize (with reason, to be sure) the slave system and slave owners. This made the conflict more personal and it made the South more afraid of and suspicious of the motives of the North.
Because the abolitionists made slavery such an emotional issue, they made compromise less likely and made war more likely. Since this war freed the slaves, you can say the abolitionists helped to free the slaves.