Lincoln outlawed slavery during the Civil War to stop Britain and France getting involved on the side of the CSA.

I think it was more to show Europe that if the North won (everyone knew they would) that the South wouldn't be able to continue export, this dissuading them from supporting the South during the war.

Like obviously the South didn't immediately stop using slaves. But it was to make foreign investors hesitant from putting money into the South and buying up plantations. Which would have been used to fund the South's army

So I guess a pre-emptive economic sanction for political reasons?

I dunno, I'm going off of memory here and it's been a minute