It seems that it just allows water to continue to flow to the bowl after the flapper shuts the main flow.
Why not just adjust the flapper or the water level to get the desired amount of water?
It’s the overflow tube so that if the filler valve doesn’t close properly, the tank won’t overfill. The excess water goes into the bowl, where the water can flow into the drain if it’s too full.
That little stream helps supply water to refill the bowl after the flapper has flapped back down. Without that, the bowl would have very little water in it after a flush.
By the way, your ball cock valve needs adjusting. It should turn the water off about an inch or so below the level of the overflow valve.
Tris
We all need our ball cock valves adjusted once in a while.
toilet humor.
That pipe serves two purposes. First, it’s an overflow, so if the valve doesn’t turn off properly, water doesn’t go over the top of the tank and flood your home.
Second, that pipe re-fills the bowl, and sewer gas trap with water. There should be a small tube from the valve pointing down the pipe. When you flush, there will be water going out that tube, down the pipe. A proper flush will pretty much empty the bowl of water due to siphoning, that little tube provides water to refill the bowl with water. This is important because if the bowl is empty, so is the trap. If the trap is empty, sewer gases can come up through the pipe and enter the room.
Thanks people. I’d forgotten about both the overflow problem and the possibility of the siphon emptying out the serpentine.