The essence of Yom Kippur is the source for Kaparah on all sins, Teshuvah (according to the Rabbanan) is the stepping stone to before we can tap into that connection that is beyond where sin can reach. There is the Mitzvos of the day, fasting, not doing work, etc. There is the Teshuvah of the day - Yom Kippur is one of the Aseres Yemai Teshuvah. Finally there the the essential bond between Hashem and Yidden that manifests itself on Yom Kippur, and comes down through the Teshuvah and the Mitzvos of the day. This is why even according to Rebi who doesn't need Teshuvah to enable the essence of Yom Kippur's bond to manifest, still necessitates the observances associated with Yom Kippur such as fasting etc. so the Kaparah can manifest itself. The Kohen Godol, representing Klal Yisroel went into the Kodesh Kodoshim alone, with Hashem, as that is what Yom Kippur is the bond of our essence with Hashem's essence. This is also the 5 Tefillos, indicating tapping into the 5th level the highest part - Yechida which is one with Hashem. Neilah is therefore the time when the gates are closed - locking us in with Hashem alone. It is not just limited to Ne'ilah - as the sages said it is a DAY that was obligated in 5 Tefilos, meaning that this Yechida connection with Hashem manifests itself throughout Yom Kippur.