Safety is one of the reasons. Heavy fouling growth reduces the responsiveness of the craft and makes it sit lower in the water. This can have serious implications in challenging weather conditions. Protecting your boat's hull is another reason. Prolonged fouling growth will damage the substrate of the hull. For example, the natural glues marine organisms use to attach to the hull damage wood and fiberglass. A third reason to ensure your boat's bottom coating is in acceptable condition is speed and efficiency, as fouling causes drag, which slows you down and increases fuel costs.
Fouling, the enemy of all bottom paints, generally has one or more of three primary causes. Animal fouling is when organisms such as zebra mussels or barnacles attach themselves to the hull. This is a frequent occurrence with boats that spend a lot of time sitting static. Plant fouling occurs when weeds attach themselves to the hull. This usually happens around the waterline. Slime fouling is when algae gathers and blooms (blossoms) on the hull. Slime fouling is the worst thing that can happen to your boat's bottom. Notice that different kinds of bottom paint have different compositions targeted at treating these types of fouling. Cuprous oxides and biocides are the two most popular chemical compounds used to combat growth. Bottom paints are also formulated differently depending on the usage of the boat. Boats that are raced, hauled and relaunched, spend considerable time sitting static, are used in warmer or colder waters etc all have bottom paints that are designed with those specific applications in mind.