Yeah, as you progress through the story you'll unlock a lot more dungeons. Once you have a couple open a new duty roulette will appear on your list, the "Leveling Roulette"(several more open up at levels 50 and 60 if you do the quests for them but the leveling one is automatic). This will randomly put you into any dungeon or story-line trial you've already completed, and once per day give you a big boost of bonus exp for completing it. This can be close to a level per day worth of bonuses, so a lot of people run it to level faster and/or work on secondary classes.

Also, Huskell missed a couple reasons people would pick specific dungeons. If youve already done your daily bonus run you might purposefully que for just your highest available level dungeons to get more exp(a bigger deal the higher your level is). Also, some people find themselves running specific dungeons trying to get gear they want(either for upgrades or to make a specific look).