Adventures are obviously central to any role-playing game. Creating rules, and characters from those rules, can and should be entertaining and fun, but up until game time, they are simply creating the role for you to play. While many Gamemasters and game designers enjoy the adventure creation process, but it is not for everyone. Some have the desire, but not the time while others may have the desire but feel they lack the skills to create a story appropriate for their campaigns. Note, I don't say they lack the creativity (although they might think so). They might not feel comfortable writing a particular story type, or feel they don't have the rules knowledge to create a level-appropriate encounter.