Typically, a game of Dungeons and Dragons or another tabletop roleplaying game relies on the imagination of the dungeon master. Like inviting friends over for a home-cooked meal, stewed over hours of notes and ideas, the DM has to carefully put together an experience from scratch.
But sometimes it’s too late for a home cooked meal—or you don’t know your guests’ tastes well enough. Sometimes you want a meal—or a game night—prepared by someone who was paid to make it for you. That’s where a pre-written adventure comes in.