A lot of the classes have an obvious "primary use" it's just some can fill secondary purposes as well.
Heavy Armor: Block; 2ndary: Lightning Rod (generally not good idea to block AND play lightning rod though)
Soldier: UP Generation; Secondary: Early blocker (some soldiers don't have Call for Reinforcements and are meant to be early dps/block)
Archer: Ranged physical damage; none of them are really good for anything else
and so on
A lot of the more recent classes are clearly dual-role though
Priest Warrior: Block + Heal
Angel: Lightning Rod + Gatekeeper
Mage Armor: Block + AoE magic damage
Some of the blacks and plats in a class can do additional roles as well though, simply due to stats and skills, like Deine can dish out massive hurt, Sophie can act as a weak field healer with AW, etc.