These nobles fought well for the nations they served in exchange for power and protection.
African nobility and aristocracy of regions conquered by foreign tribes or entirely foreign societies sometimes remain in place, occasionally even empowered over local rivals who had sided against the conqueror. These empowered men are of good standing, and are used as local administrators, or taken on campaign as a benefit to the local troops they lead.
The bodyguards of these men come from the African nobility of the many Numidian tribes and are equipped with good armor, a sword, javelins, shield, and a good quality helmet. They fight well as long as they are lead by a capable general, and will give a hard fight to any enemy because of the long tradition of African skirmishing cavalry.
Historically, many African nobles campaigned for many people outside from their culture, such as Carthaginians and Romans. We see this mostly during the Second Punic War when tribes with a common culture chose opposite bands. We hear of names like Narawas, Syphax, or Masinissa, who changed from Carthage to Rome and indirectly lead to the victory of the Romans at Zama.