A 4x4 snorkel is useful for off-road vehicles that operate in dusty, dirty, wet, or otherwise treacherous conditions. It can allow for cleaner air intake, which can improve gas mileage slightly as well as power output of the engine. The 4x4 snorkel can also help keep dirt and water out of the air intake, thereby improving the efficiency of the engine. Vehicles that routinely do water crossings will benefit from the snorkel because water is less likely to enter the engine, though driving through deep water can still damage an engine regardless of the addition of a snorkel.
While you can install a 4x4 snorkel yourself, it is wise to have a professional do the job. A professional can ensure that the system is sealed properly and body panels are not damaged excessively during the installation process. The system needs to be installed correctly to ensure proper air delivery to the engine, and to ensure no leaks or cracks develop over time. The snorkel itself will need to be positioned properly outside the vehicle to ensure the driver's view is in no way obstructed during normal driving on- or off-road.
Dust and dirt are heavier than air, of course, which means if air intake can be performed higher up off the ground, less dirt and grit is likely to enter the air intake system on the vehicle. A 4x4 snorkel extends upward from the engine, sometimes above the height of the vehicle's roof, meaning less dirt and dust will enter the air intake than it would if the intake is taking place under the hood of the vehicle. More dirt and grit can get trapped underneath a hood of a vehicle, thereby raising the likelihood that more dirt will get sucked into the intake system.
The 4x4 snorkel will also promote an air ram effect, meaning that more clean air will be pushed into the engine during combustion, which will promote more efficient combustion that improves power as well as gas mileage to some degree. The gas mileage improvement may be nominal, though on some engines that are tuned properly already, the gas mileage improvements may be more noticeable. The power difference, too, will be affected by the overall efficiency and condition of the engine in combination with the use of a 4x4 snorkel. The biggest benefit of the snorkel is keeping foreign objects out of the engine, which will help prevent excessive or premature wear of engine components.