The block may be the single most useful object on a sailboat or any boat where a human cannot physically haul or lift an object. A block can be used thousands of ways, which include raising sails, lifting engines for repair, trimming sails, lifting the dingy, and can be used to hoist an injured crew out of the water.
Blocks are traditionally used in a vertical sense to lift objects straight up, but you can also use them to haul things sideways and redirect force towards an angle that you may not otherwise be able to access.
Next time you need to move or lift an object, ask yourself. "Could I use a block to do this?"