Expert dungeons generally are easy so you can practice your jobs at the level cap in a combat situation. When you get to 60, and you finally get the capstone ability that completes your job, throwing you into a dungeon with a significant chance of wiping you is counterproductive. But you can't just use a lot of abilities on a practice dummy, like blackest night. So you have relatively easy dungeons that let you try out stuff in combat. Its nice if you have't played a job in a long time either, or your job just got radically changed.