The decorations seem like an elaborate form of the tzitzit that Israelites wore on the hem of their four-cornered garments. If this analogy is valid, one might say that the high priest showed his extra rank by means of his more ornate garments.
The question is why the decorations on his robe took this particular shape (pomegranates also figured in the decor of the Temple). The pomegranate decorations were probably pom-pom balls of coloured material. Was their significance linked with the many seeds inside a pomegranate? Or were they merely a decorative feature?
One could dismiss them as mere decoration if not for the bells with which they alternated.
Bells have a practical use and many commentators think that having them on the priestly robe was to ensure that the people could hear the tinkling when the high priest moved, especially when he was inside the Holy of Holies on Yom Kippur. Can we link the Yom Kippur ritual with the pomegranates too?
Possibly the bells were to inform the people that the high priest was praying for them and the pomegranates symbolised that he was praying for a people as full of good deeds as a pomegranate and as blessed with children as there are seeds in the pomegranate.