It’s not aesthetic, though I’m not completely certain what the consequences are.
Usually the cited reason is the blade causes less suction. With knives, this tends to get presented as something that makes it harder to pull a blade back out of someone, though, really, if you got it in there in the first place, through intact tissue, pulling it back out shouldn’t be much of a barrier.
If it does actually generate less suction, it probably means the wound can’t seal around the blade to reduce bleeding, if it’s left behind. (Incidentally, first aid for someone who’s been stabbed is to leave the knife in the wound. Pulling it out will dramatically increase the blood loss through the wound, and you can kill someone that way while trying to help them.)
It may just be to increase the blade’s ability to wound while avoiding serration, and the difficulty with trying to sharpen or hone a serrated blade.