The long vent hose is usually directed down to the road and away from hot exhaust pipes, cylinders, and away from your crotch (which to me is the most important part of my body). The tanks have to vent to allow for the expansion of the fuel and the ingress of air - if no air can get in you'd get a vacuum forming inside the tank and no fuel would gravity feed to carbs/injection system after a few kms.
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