Why if we do Hatarat Nedarim Erev Rosh Hashana must we say Kol Nidrei and conversely, if we say Kol Nidrei why must we say Hatarat Nedarim?

The most common answer for the first question is that Kol Nidrei refers to the vows of the community, while Hatarat Nedarim takes care of personal vows. On another level, Kol Nidrei (according to one Nusach) frees us from future vows only if the condition is forgotten later, while Hatarat Nedarim is for all personal vows.