Lots of additives hold a certain ‘snake oil’ appeal to some folks I suppose. I don’t think that is the gist of this thread however.
Where small engines are concerned, I’ve certainly seen more harm done by tinkering with carbs when proper use and very simple maintenance (like good filters and water displacer additives) will prevent 99% of the problems. (Water creates ‘solids’ in aluminum and zinc carb bowls when it sits in there and causes corrosion by-products for example.)
Suggesting that water in fuel is not much of a concern however unless it is barged denies the reality that cold weather brings to the picture. The water that gets in fuel, even the stuff which is barged in, is largely atmospheric in origin as it sits in storage. The same is true of many vehicle-mounted fuel tanks. Most people have no idea what’s actually in their tank and even fewer drain and clean them. Some of that stuff that accumulates in the tank will eventually find it’s way beyond the filters and into the carbs or injectors. Some fuel additives can be useful periodically for cleaning out the stuff that can cause rough idling. SeaFoam seems to work quite well in this regard. It might not be better than a 10 minute carb job on a simple lawn or garden implement, but it sure beats spending a couple of hours tearing into machinery that is still in need of chemicals of one sort or another for proper cleaning, or could have been avoided in the first place by the use of a proper additive.
Obviously nothing you add to the fuel or oil will repair worn rings or cylinders, or repair leaky valves.
