Grenadiers were originally the soldiers in charge of throwing grenades (hence the name). They were chosen for being tall, muscular, and fearless - not only did they need the strength to throw grenades far enough to actually injure the enemy - they had to be brave (or stupid) enough to stay in the front lines of battle to do it. Grenadiers were also called upon to be fast, effective assault troops - helping with things like storming breaches during sieges. (You know, all the standard easy jobs that are likely to get you killed.)