Without fail, within 3-4 weeks someone will post that their bike is having difficulty starting, and that fuel is leaking out of the bowl overflow, or their exhaust is black and stinks of gas, or one of their plugs is fouled, or one of their headers is cold. Someone will suggest that the problem is a stuck float valve, or a bit of crap in the float needle. I will recommend that the petcock be turned off, and three or four people etc etc...
About once or twice a year someone will post, tearing their hair out, that fuel is POURING out their airbox, exhaust, crankcase breather. We have a fellow on this board right now who is ready to sell his freshly-rebuilt bike for chump change because this has so freaked him out. A float valve stuck open, and allowed the gravity-fed tank to drain into the engine. Why? Because the petcock wasn't turned off. A dead simple problem with an even simpler solution, and he's ready to throw up his hands in despair and disgust. (This is not a condemnation of that individual, but rather an extreme illustration of how such a simple system can be so misunderstood.)
I've posted on a lot of boards for a lot of years, and it's the same thing again and again. The float valve is a FLOW CONTROL valve; the fact that it is (nominally) capable of Tight Shut-Off is almost secondary. It is a back-up in case you forget to close the actual Shut-Off Valve, which is the petcock.
A comparison - the primary means of slowing the vehicle is the braking system. This is what the manufacturer intended that you use to slow down. The engine will also slow you down. How many people use JUST engine braking to stop at a light? How many racers use JUST engine braking at a bus stop - or ANY corner, for that matter? Did you know that using JUST engine braking before a corner will increase your odds of crashing? Even that brief moment of braking warms the tire, giving you better grip. This is completely aside from the possibility of the clueless cager behind you driving up your ass because the brake lights didn't come on!
The float valves will prevent fuel flowing into the engine - unless they don't. That's the job of the petcock. Use the damn thing!
