Gasolines are made up primarily of aromatics (toluene, xylenes), paraffins (isooctane, butane, heptane, etc) and olefins. Aromatics and paraffins are very stable but olefins are like molecular nymphomaniacs and want to react with everything. They are the precursors to gums and varnishes and that's what Stabil, Seafoam, etc. stop the reactions of. The good news is that there are very few olefins in gasoline any more and stabilizers are not as needed as they once were (if at all). Like Mike says: use them if you want to, but you probably don't need to. Someone here will probably disagree, but ethanol doesn't contribute to gums or varnish - it's the same stuff that's in your 10 year old open bottle of Jack Daniels.
