Firmly in the camp of "make it go faster and grip / handle better for road usage"...
That said, there are still things that might make a car faster, but wouldn't personally do... like the blob vent vs other models, and especially dislike the look of it on classics -> however, others seem to like or not mind the look.
Few things I have come across that made me scratch my head alright:
1. Fitting a race / paddle clutch on a daily driver with stock (or close to stock) power that is never tracked... total waste of time, no "aesthetic" benefit, no performance benefit given the intended use of the car... just a total pita to drive.
2. Along the same vein... mods on a daily without benefits only draw backs: a) fitting seriously hot cams on a daily that isn't used on track. Ruins the car as it won't idle and power band shifted to an area you don't use. b) same as above with a turbo that is way too big for a daily (no power and only lag till 5k and then mad power for 1/2 second till you hit the rev limiter). c) (overly) lightened flywheel... a modestly lighted flywheel can be ok as gives a bit more responsiveness, but lighten it too much and the car becomes a pig to drive as needs about 2k revs to take off without stalling it (which sucks on a daily).
3. Some mods that you thought were, or might be, ok when you clicked that "buy now" button, but seriously disappointing or too hard to live with: a) too loud an exhaust (fun initially... until it wears on you and you wish you could go somewhere without needing ear-defenders), b) cheap coil-overs... cheap pads... cheap tyres... "cheap anything" that ends up ruining your experience (of safety).
4. Modding the car for track use... but only doing half the mods in the wrong order... so... for example: up the power, but not sort the braking / tyres / cooling / oil & fuel starvation during hard cornering... recipe for disaster as in best case scenario car is still slow, or worst case scenario... accident, or engine failure.
5. Bad (re)maps... or no (re)map after fitting certain mods... = engine failure. Seen it too often.