Any device that deals with air has to deal with the things that are part of air like dust and moisture. The air compressor manages to keep most of the dust particles out of its system by way of air filters. However, moisture, which is the same size as air, cannot be kept out by way of filters and there is no way to keep it from entering the compression chamber of the pump as well as the tank.
What is possible though is to remove moisture content from the tank, once it is stored. This particular task is accomplished by the dryer that is part of every air compression system. Compared to other parts in a compressor, a dryer may seem like a non-essential component of the system. This is true because, even if it is not there in the system, you can still get your job done. If you thought, the same way, then you are not looking at the big picture here.
With an air compressor, we are looking at an expensive piece of machine, and by extension, you expect that to last a long time. If there is one thing that can cut down the life expectancy of your favourite compressor that would be rust, a result of metal reacting with moisture. Rust is a powerful agent of destruction, and grand ships have fallen victim to this simple metal ailment.
A good dryer would ensure that the moisture in the compressed is removed, directly contributing to longer life to not just the tank, but the compressor as well as all the air based tools that will get to use dry air to get the job done. The only to remember while buying a dryer is that its capacity should match with the capacity of the tank of the compressor. While getting a bigger dryer is not a problem, getting a low capacity dryer for a large tank is as good as not having it there in the first place.