Hello. The knife, if you just slash once, usually only cuts down to the muscle. The scalpel could cut all the way down to the bone in a single slash. Or, if you ever figure out how to implement organs, the scalpel could be used to cut down to them. Kind of like surgery, except not in a good way.
Oh, wait... that's actually a good idea. However, there is one problem with the logic of this. Scalpels tend to be much, MUCH sharper than knives, because knives are meant to cut stuff like meat, and scalpels are meant to cut flesh, and human flesh is tougher than many types of animal meat and/or skin, so scalpels would need to be sharp.