Hinata's purpose is to be the only person who was never a b**** to Naruto and to have him as her source of insperation and power. Naruto influences her and she does more for him then Sakura (in terms of being a better person towards him). Sakura has more of a purpose because she's the main female character of the series. Sakura's roll however is more harming to Naruto then Hinata's even if she is only a supporting character in the series. You say she does the same shy bs, well she swallowed that when she jumped in front of pain ready to fight and die for Naruto and confessed to him. Sakura dose the same needy thing with Naruto and the inability to actually help him. Sakura's purpose isn't better, there is no better purpose only more significant. In this case Sakura's purpose is significant in being an overly dependent person on the protagonist, Naruto and her inability to do anything at the moment about the two people closest to her. She loves Sasuke too much, still more then Naruto but she does in fact care about him, but not enough to truely let go of Sasuke. Hinata's significance is still as a supporting character but as demonstrated with her actions against pain, her significance may change to a more significant one to aiding Naruto, as it stands she's more helpful to Naruto then Sakura is (not neccisarily in terms of skill but affection).