yeah, goliath was supposed to have been from a leading family. and shield bearers were very common for a long long time. there are several ways it can play out:

the shield bearer may serve as an attendant, helping the warrior to stay fresh by lugging the materials for him. this was generally the case with greek shieldbearers, and these usually stayed away from battle, though they might be expected to give assistance to their warrior if he were wounded.

the british chariots actually operated in a similar way at times, delivering a warrior to the battlefield, and then serving as a lift out of the fray. celtic attendants might serve in battle as well, such as the three-horsemen system among some celtic cavalry, where one noble would have two horsemen as reserves and auxiliaries to his own actions as the lead warrior.

less militarily active but to some extent comparable are the helots in the Spartan army, who at times could take active roles in battle. in hellenistic armies, many soldiers had paides, sons or--more often--young slaves. These served as attendants, and would carry some of their equipment before battle. There's little evidence that they participated in pitched battles, but may have served as light troops in regular patrol activities and minor actions though, considering the value of a healthy young male slave or son, I doubt they would have been put into too dangerous a position with much frequency.