If you're a tank and you level up a dps class then you get to see what it's like from the dps perspective. You learn what things tanks do that get on your nerves.

An example is if you level monk or dragoon and the tank keeps spinning the enemy around because they think moving around helps them avoid normal attacks. You run around, wishing the tank would stop moving so you can properly hit the enemy.

Experiencing the perspective from all points of view makes you a better player overall.