My grandpa(age86) always shuts the gas off on his old tractors, and my dad was taught the same thing. Now me & my dad are wondering do we really need to do that. Where does this idea come from?

My husband and I collect old farm tractors..mostly old Farmalls..but we do have John Deere and old Fords..and we were looking for the model kits for old tractors. Like the kits you put together yourself, like the cars and airplanes. Any help in locating them would be greatly appreciated! Thanks!