Pre-cut, or pre-slotted nuts are nut cut for the width of the neck. You still have to fit the width and you still have to fit the overall elevation, and you still have to fit, usually, how it fits into the slot. The width part is pretty easy really, a non issue if the neck is unfinished - just sand it flush. On a finished neck, you can fit a pre-slotted nut....its a bit more work, but not a lot. You just have to work it a little to get it just right. A sharp scribe works for this, as well as good eyes and a gentle touch on the sandpaper. You can choose to sand it flat - on the guitar - with a sanding stick, and do a minor finish touch up, or sand it just below the surface off the guitar and fill the slight intend back up with finish. Or you can take your time and have it fit "just right".
The wild maple tele I've got... on the goncalo neck, I put a pre-slotted nut. First I had to work the sides, so it fit in the slot, but still snugly. Then I worked the bottom of it to get the slot elevation just right - I never touched the string slots on that nut. Then I fit the width, keeping the slots centered. Hogged off 99 percent of what needed to be removed, then used a sanding stick to finesse it level. That resulted in a few really small scratches on the finish. They buffed out, cant even see them. But I could have given them a quick drop of lacquer and fixed that all up perfecto. HINT: Nail polish is lacquer. If you ever need to make a really small and quick oopsie fix, even on poly... hit it with a tiny drop of clear nail polish and let it dry up for a while. Level that and polish it out... its pretty good for that. That assumes you're not really into "finishing" and dont have anything else.