"A write up on this topic could enlighten us."
I'll give it a shot.
Gas evaporates quite easily. In the past, these evaporative vapors were vented into the atmosphere via a vented gas cap. Approximately 20% of all hydrocarbon emissions from a gasoline vehicle are from the gas tank. In 1970, laws were passed prohibiting venting of gas fumes from the tank into the atmosphere. An evaporative control system ("evap") was developed to eliminate this source of pollution. The function of the fuel evaporative control system is to trap and store evaporative emissions from the gas tank and carburetor. In an EFI engine, the throttle body has no bowl or collection of fuel to deal with. This is accomplished by use of a sealed gas cap and a charcoal canister to trap the fuel vapors. The fuel vapors adhere to the charcoal, while other fumes pass right through, until the engine is started, and engine vacuum can be used to draw the vapors into the engine, so that they can be burned along with the fuel/air mixture. This sealed gas cap is so important to the operation of the system, that a test of the cap has been incorporated into many state emission inspection requirements. Gas tanks were also redesigned. The tank has to have space for the vapors to collect so that they can then be vented to the charcoal canister. A purge valve in the canister is used to control the vapor flow into the engine. The purge valve is operated by engine vacuum in a carbureted vehicle, or via the ECM in an EFI controlled engine. With a carbureted engine, the canister usually has four connections. The first is for the fumes vented from the gas tank. The second is a line to the carburetor bowl. The third is the purge connection that is attached to a ported vacuum source on the engine. The fourth is a fresh air intake so when the purge control opens the valve to draw in gas fumes, a fuel/air mixture is available and to also allow air to enter the tank as fuel is drawn out. When used with an EFI engine, the second connection to the carburetor is obviously not needed, so a three port canister is usually used (tank vent, ported vacuum source, and fresh air). In addition, the purge control valve may be electronically controlled by the engine ECM rather than plain vacuum operated.
Now on some vehicles, including our beloved CJs, this system also includes a check valve to prevent gasoline from exiting the tank through the tank vent connection and spilling into the canister's "tank" port by placing a check valve in that line between the two. In the event of a rollover (or severe off camber incline) this check valve closes and blocks gas from exiting the tank. This is not required for proper functioning of the evap system, but it is a safety feature any off roader should have in place.
Now if your state doesn't require any emissions testing, then you could theoretically just get a vented gas cap (good luck with finding that), cap off the tank vent and the carb vacuum port, and eliminate this evap system altogether. But beware - you will deal with the smell of gas fumes venting through the gas cap, especially in warmer weather. This will also decrease fuel mileage as you are losing burnable gas fumes into the atmosphere that you could be drawing into the engine instead.
One common problem with this system is that the purge valve goes bad (stuck open) and engine vacuum draws fuel directly into the intake system. This enriches the fuel mixture and will foul the spark plugs. Most charcoal canisters also have a filter that should be replaced periodically. In a sealed type canister, the entire unit must be replaced when the filter is no longer functional. This system should be checked whenever fuel mileage drops, the engine is running too rich, gas fumes are detected (usually by smell), and/or the canister leaks fuel. Another common problem is that the purge valve can become stuck shut, preventing the engine from drawing in the collected fumes, and possibly causing premature failure of the filter (think "waterlogged" but with gas), or perhaps even a vapor lock condition in the gas tank (the fresh air inlet is blocked).
In my case with an EFI engine, I have a three port canister (tank vent, fresh air, ported vacuum). The ECM that came with my engine does not have electronic evap control. So a ported vacuum source on the throttle body handles the job. I also included the rollover check valve between the tank's vent and the "tank" connection on the canister. It doesn't really matter where you mount the canister. It is usually in the engine bay. But if space is an issue, it can be moved. Mine is mounted at the rear driver's side behind the inner fender well (where you access the bolts holding the taillight on. It is just below the check valve mounted above it. This was done due to space limitations under the hood with my dual battery, OBA pump, SBC with headers, alarm siren, secondary fuse panel, extra relays for the dual electric fans, etc. I then have a metal line (piece of metal fuel line) with a short length of rubber vacuum hose at each end travelling along the left side frame rail to the engine bay to connect to the throttle body.
If someone has more to add, please do.