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In combat, a shield can go a long way towards keeping a sword or a spear away from your vital organs. Adventurers who carry shields are often front-line fighters who take their roles seriously and don't want to deplete the healing resources of the party by taking foolish risks. Who am I kidding? I've never met a player who didn't want to take foolish risks in combat! Shields just make it look like you are serious and increase your armor class by 2. Use them.