Depends on which ones you are talking about.
Some are just bottles of alcohol, and put water in suspension.
Others contain detergents, which then clean carbon in the combustion chamber, on the piston rings, and valves.
I remember back in Dec '98, I had to move from Denver to NYC, and had rented a Ryder rental truck. It was only 2 years old, but had a bunch of miles on it, and was having an idle issue, and was sluggish. So I tossed 2 full bottles of Techron Complete Fuel System cleaner into the tank, and started heading east.
Truck ran so good after the first tank of gas, I was quite shocked at how well it idled, and how smooth the throttle became. Gas mileage went up too, which helped a lot.