Javelin is a generic term, pilum is a term for a specific kind of Roman javelin. The replacement is in that Roman soldiers started carrying these darts instead of pila - while pila were more general-purpose and heavy-duty weapons, these light plumbata were much more of a harassing tool that a soldier could carry more of.
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Cost savings measure. In addition to being heavy for the troops to carry, it was expensive to make.
Javelin is a generic term, pilum is a term for a specific kind of Roman javelin. The replacement is in that Roman soldiers started carrying these darts instead of pila - while pila were more general-purpose and heavy-duty weapons, these light plumbata were much more of a harassing tool that a soldier could carry more of.