I had also some concerns about this. I play as Gladiator and doing my first 3 dungeons I realized that always the need button is greyed even on items that are perfectly fine for my class.
Example: A chest Drops with +3 Vitality, +3 Strength, +2 Dexterity and +1 Determination

Lancer and Archer could need it but my need button was grey..as far as I understand
Vitality increases my HP and HP regen
Strength increases the damage I can mitigate through Blocks
Dexterity increases the chance to evade and parry and block?
Determination increases the heal I receive?

So, why I don't have rights to roll on this chest? This is not a qq post, I really want to understand where I am mistaken. I never been a ninja in an MMO but I would like to know why this item above was not for my class. Thanks