A fuse block is a power distribution device. As Jeff said, if you want to add electrically powered devices, you would use a fuse block if you don't ant a rat's nest of wires emanating from the terminals of your battery. You would run two large gauge wires (-and +)from the battery to the fuse block, and then individual smaller gauge wires from the fuse block to each accessory. The benefit of the fuse block is not only less clutter at the battery, but each individual circuit from the fuse block is fused for protection. Also, some fuse blocks contain relays for powering high current devices like air horns, heated gear, etc. Also some fuse blocks have provisions for constant power with ignition off, and/or switched power.
I like Centech stuff because they are nice and small, and well built.
Another very good but pricier company is fuze blocks: