The main way a gas stabilizer works is by slowing the rate at which hydrocarbon compounds in gasoline undergo oxidation reactions with water and other atmospheric molecules. If molecules in gasoline become too oxidized, they become too heavy to remain in solution and fall to the bottom of the gas tank as a solid mass. Engines cannot burn the oxidized compounds as fuel, and the masses can cause clogs and other problems in the fuel system.
Most drivers do not need to use gas stabilizers because most gasoline does not become seriously oxidized until over one month after it was refined. Because the majority of drivers take much less than one month to use a full tank of gasoline, there is little risk of oxidized fuel causing problems in the vehicle. However, in vehicles or equipment that sit idle for months at a time, such as snowmobiles, lawnmowers and small boats, any unstable fuel left in the tank at the end of the operating season is likely to become seriously oxidized by the beginning of the next season.
While oxidation has always been a problem for gasoline, the situation has gotten worse since fuel companies started incorporating larger amounts of ethanol into gasoline supplies. Ethanol is less dense than most of the other compounds in gasoline, so it eventually travels to the top of the mixture where it interacts with the air. Ethanol is better at mixing with atmospheric water molecules, which means that ethanol-rich gasoline products absorb even more of the oxidizing compound than those that are pure petroleum distillates.