To ease the fear of the Southern slaver owners that Lincoln would not interfere with their slaves, as he was not legally able to do in the first place.
What Lincoln ran on and intended to enforce was stopping any further expansion of slavery.
But you know this and these attempts to attack Lincoln for his 'racism'and his attempts to keep the Union together by ensuring the South that their constitutional rights would be protected under his administration, are simply attempts to muddy the water.
And yet the southerners weren't interested. Pretty strange doncha think, especially if, as you say, it was "all about slavery."
What Lincoln ran on and intended to enforce was stopping any further expansion of slavery.
Yet he was perfectly content in enshrining its permanent existence into the constitution in a way that would have undermined the Somersett argument with far greater damage than Dred Scott or anything else could have done.
If you really believe that, then I've got a bridge I'd like to sell you.