You usually rest a steak because it will keep cooking after you pull it off of the heat source. This lets you cook it at high heat and let it coast to the desired doneness. If you cook to desired doneness and slice immediately, you end up with a larger ring of overcooked steak around your central region of optimal doneness. Because this method cooks slower, you can stop closer to desired temp. If anything, the sear at the end has some potential to increase that region of overdone again, so slicing will only stop that undesirable process.