Here you go. Here's what the much lamented charcoal box is for...
When you put gas in your tank, there is some air in there with it. Gasoline has a strong tendency to evaporate. The vapors are trapped in the charcoal box. When you start your engine, air is sucked through the charcoal box, into the gas tank, recovering some of those vapors.
As far as emissions, it doesn't have anything to do with your exhaust, except that it gets in the way of some catback setups. It keeps your evaporating gas from exiting into the atmosphere.
When you put gas in your tank, there is some air in there with it. Gasoline has a strong tendency to evaporate. The vapors are trapped in the charcoal box. When you start your engine, air is sucked through the charcoal box, into the gas tank, recovering some of those vapors.
As far as emissions, it doesn't have anything to do with your exhaust, except that it gets in the way of some catback setups. It keeps your evaporating gas from exiting into the atmosphere.