In the Greek army, shield bearers armed with swords fought alongside phalanxes to protect their unguarded left flank. Greek shield bearers were unarmored except for a helmet, and carried a shield and sword (or in some cases, a spear instead).
The Spartan hoplites developed larger hoplite shields when the Macedonian phalanx came up. A shield bearer carried it on the march. In combat he served among the light armed troops, usually as slinger.
But when killed the morale of the phalanx was crushed, as well as when the standard-bearer died.