It adds tons of features to the Minecraft server. The whitelist, bans, admins, groups, kicking, playerlist, motd, commands, preveligies, chat tags and the most important thing, being able to install and download plugins. (Towns, autobackup, economy system, creeper nerf, admin tools, backup tools) Look at the server like a car, the car can roll without an engine but with an engine it can roll itself. The engine is hMod.