Removal of ban against sinners:
Prior to Kol Nidrei the leader of the congregation, together with two other people remove all excommunications and bans that were placed upon any of the worshipers in order for all Jews to be able to pray together with the congregation. This holds importance not only for those that were banned but also for the congregation itself, being that any fast which sinners do not partake in is not an appeasing fast, as is learnt from the incense offering which included also a foul smelling spice.
 Ruling in Shulchan Aruch: In the Shulchan Aruch ibid Admur writes that it is done on the night of Yom Kippur prior to Davening. Practically however in the Siddur it is written to be done before Kol Nidrei which takes place before sunset.
 Thus in order to give all sinners the opportunity to pray with them, a nullification of a ban must be made.