Fill Valve and Overflow Pipe

I understand the overflow pipe prevents the tank from overflowing.
The fill valve has tube that flows into the pipe while filling. Since the overflows empties to the drain, what then is the function of water tube from the fill valve?

When you flush the toilet, the trap in the toilet can siphon dry allowing sewer gases into your bathroom.

The fill tube ensures the trap is refilled after every flush to stop this happening.

The fill valve main output refills the tank, the tube in the overflow refills the bowl. If it is calibrated well, little ends up down the drain until you again flush (older ones overfilled the bowl a lot). The overflow also provides protection if the valve doesn't shut off - it lets the tank overflow into the bowl and down the drain instead of running out into the room. A well performing toilet siphons out nearly all of the contents of the bowl during the flush (otherwise, you'd have some potentially nasty leftovers!). You need water back in the bowl to seal the trap and for the next time it gets used - that comes in from the overflow tube during the refill.

Ahhh… and I’m thinking the new Toto does not fill the bowl as much as the Kohler it replaced (as with my remaining Kohlers). All along I was wondering why with the Kohler the bowl appears filled yet the water keeps running/filling, so this is in fact waste/inefficiency then.

Now it seems funny that folks (me, at least) compare the Toto and it does-not-perform-as-well as the builder grades, when the exact opposite is true. I was beginning to think there’s something wrong, that this Toto Supreme needs to be calibrated/adjusted when it’s doing/functioning exactly as what a good toilet should.

Thanks, guys. From perusing this site, I never thought I’d be learning the crap, I mean science in toilet design.

toilet

A toilet flushes by the siphonic priciple. Unless the requirements are met when the toilet is flushed, it will either not flush or flush improperly. The refill tube was to insure that the bowl was full of water when the flush began, so ALL the water from the tank could be used to produce the siphon. Some bowls, if the refill tube is not operating properly, will use most of the initial water to refill the bowl, and then the remaining water will not be adequate to create the siphon, and then it has to be flushed a second time.