In the past, fuel had so much contaminants in it that when it dried it left residue or a gummy layer. This would eventually mess up the carb ports/needles. Thus the product "Gumout".
If you shut off the fuel, and run the engine till it dies, then most of the fuel is out of the carb, and there will be less of the residue.
Actually modern fuels are much less of a problem in this regard.
Had a local lawn mower dealer that would recommend a carb rebuild on EVERY mower he worked on. Once, I had a fairly new mower that had the fuel tank just replaced by factory recall and I put fresh fuel in it. Took the mower there for some regular maint. and they said the fuel was bad and the carb needed cleaning. Showed me a glass jar of really brown gooey fuel. I KNEW They were lying because I had just replaced the fuel in that tank the day before. They are now out of business.
But back to fuel gumming up the carb. I had a 62 Catalina with 389 engine and tri-power (three 2 bbl carbs) about 10 years ago. I raced it, and stored it each winter. Sometimes just during the summer the gas would evaporate several times a year. After I owned the car for about 5 years, I took the carbs apart assuming they were "gummed" up. They were clean as could be. I didn't use any fuel additives, just bought quality modern fuel.
Today's fuels, especially with the ethanol in many of them, will keep a fuel system really clean. Many fuels have an added fuel system cleaner on top of that. That is why it really isn't that necessary to add SeaFoam or similar additives all the time. It won't hurt, if used according to the instructions, but it just isn't really necessary.
Fuel injected bikes, like the GoldWing don't have fuel bowls and they don't evaporate a lot of gas when shut off, so this isn't a problem for those bikes, and why they don't have fuel shut-off's anymore.
Oh, and that shut-off also could prevent other problems in older carb equipped bikes, if the fuel float would stick, the bike could leak a LOT of fuel, not good if you store your bike inside where it is heated and the furnace kicks on!