Wet sprinklers systems are the most common and simplest. These systems have water in them at all times. When a fire occurs, the system will activate, causing the sprinklers to open and water to flow.
Wet pipe systems are:
Reliable - a simple system means less components to potentially malfunction.
Easy to modify and maintain - in order to work on a wet system or to make alterations, the water is shut off and the system drained. Then the work can begin. When the work is done, the system is pressure tested and restored.
Affordable - being that these systems are simple in nature, installation and maintenance costs are low.
Not suited for extreme cold, as the water inside the pipes can freeze.