Air flows into the can through the hole while beer flows out of the can through the mouth. You can get a smoother pour without the second hole but you have to be careful not to pour too fast so that the beer flow doesn't block the entire mouth, but that only works if you pour it into a glass. If you're drinking straight out of the can, your mouth covers the entire mouth of the can and no air can flow in at all.
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SuperNiceDog didn't have to reconcile his name...
But Dauntless Hunter is now MrGreenonion