Your engine is filled with precision parts that each have a very specific job. In the tight confines of your engine, these systems work side by side, often complimenting each other. In order to keep the fluids or gases in one system from invading another, even if they perform complementary functions, a division is necessary.
Made of metal, carbon fiber, silicone, and even paper, gaskets work like fences to keep areas tightly sealed, often sandwiched at the seam between two systems. For example, your head gasket sits between the engine block and the cylinder heads, working to keep the coolant that reduces engine temperature from interfering with the work of the pistons should the fluid leak. It also keeps the gases from the pistons from leaking into the coolant system.
Even durable materials fail eventually. Your engine will need gasket replacements from time to time. Weaknesses in your gaskets may be discovered during the course of a routine inspection, allowing you to have it replaced before it fails.