Whitebeard being under Rocks and ultimately being truly loyal to him is to establish greatness of Rocks.
This is similar to Roger having "dark king" Rayleigh, Whitebeard having Oden under him, Luffy having Zoro under him. They exist to hype up the greatness of the captain himself.
Yeah, but you don't see Roger being an underling to another man. Yes, great men have great followers, but I would've expected from the great Whitebeard to be atop, rather than being a lackey in his mid 30s when already having all of his arsenal (fruit, blade, ACoC). It is a loss of aura for the world's strongest man and king of the seas who stood atop all the pirates and ruled in front of the throne. It would've preserved his aura way better if he was a leading player in that flashback, rather than the era revolving around Roger, Garp and Xebec as the three legends mentioned.