Land of Legends
Mercenary is a broad term that describes those who sell their body and services in combat for money. Fighters, Sorcerers, Archers, and horsemen are all commonly found as Mercenaries. However even some Rangers and former soldiers may choose to seek this life. Only Guardians tend not to become Mercenaries as mercenaries often don’t get to pick which side they fight for in a conflict.
Mercenaries are common because maintaining standing armies is expensive and it’s much easier to simply hire those with the training to fight whenever the need arises. Also things like caravans and traveling merchants need protection from thieves and bandits and the cheapest way to do so is to hire experts in arms.
While you can be trained in what is needed to become a mercenary, it is far more common to see someone trained in some form of combat become a mercenary then for someone to set out to be a merc.