You get bonus EXP and tomestones from MSQ roulette, including Genesis if you're at endgame.
Roulettes are a good way to level up in general - all reward you with varying amounts of bonuses (EXP, tomes, GC seals, materia, gil) and the rewards scale with your level.
There isn't anything you really need to do to prepare for them - just use your preferred class and current gear, and your stats will automatically adjust to the dungeon. You won't get anything above your current level.
You mentioned your character is already at Lv70, but there is a specific Lv70 roulette (for endgame dungeons that reward Mendacity and Genesis tomes) and also 'Experts', which isn't as daunting as it sounds - it's just the most recently released dungeons, currently the Burn, Arboretum and Ghimlyt Dark.