What is the Adventure Book for?

[ZombieDad]

I'm hoping the answer to the adventure book question will answer my deeper question:

I'm wondering how the campaign/buff up your character will work with a mission order that doesn't appear to matter. The description mentioned that you can roll a dice or choose a mission. Doesn't that mean the missions are of the same difficulty? If so, won't missions become easier as the game progresses?

Seems like in games that advance your character, the order of missions matters. Descent seems to have an order to the missions and so does Pathfinder Card Game.