In Guild Wars 2, just about every profession can be any class, as long as it’s setup with the right skills and weapons. Support in particular is when a profession has a skill set that is geared towards healing allies and removing status aliments. [A list of all GW2 classes]
This class is extremely useful in PvP because having good healing in a group can win a fight if the opposing players do not have a healer.
This is not all that a Support player has, they can also give some buffs to allies, and some debuffs as well, depending on the skill setup. [Guild Wars 2 main page]
Support is the most crucial role in group based PvP in Guild Wars 2, without it teams can be extremely vulnerable. The team with good support type players is the team that wins.
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