Main Tanks are in a unique position during Raids and Trials as they have a perfect view of everything that happens in the fight. They can clearly see what the other 7 members of the party are doing. Main Tanks generally stand in front of the boss doing pretty much nothing most of the fight so there's plenty of opportunity to absorb mechanics quickly. In this manner I find that learning an entire fights mechanics goes far quicker on Tank. If there's no first-time completion bonus I'd expect the main Tank to know the entire fight.