Simply put, they are duties that you go through while levelling through the story. As such, they are in the levelling roulette. I for one really am happy when I get Ifrit, or Garuda for example, in my levelling roulette, as it makes for a very quick and smooth roulette. And more importantly, if you end up in one of those, that most likely means that there was someone queued specifically for it that needed it at that point in time. It is very useful for new players doing those duties for the first time to have this way of getting extra players for it.
If there's one complaint I have about roulettes, it's that duties you have unlocked but haven't yet completed can also show up. Like that one time I was intending on doing The Singularity Reactor with friends some time later, but I got it in my trial roulette that day. I felt very awkward and unprepared. This is also how I ended up doing my first Zurvan. After months of putting it off, it showed up in my trial roulette one day.