The roulette is there first and foremost, not for variety, but to match players into duties that they otherwise would not select. For this risk of getting stuff they don't like, they are given a bonus reward, once a day.

The roulette is there to help shorten the queues of people who need to complete specific duties. People are mistaken to think it is there to give you an edge on leveling. No. All that bonus and all that variety is a by-product of putting you where you are needed.