The torah tells us to bring korbanot to atone for certain sins (when bringing korbanot is possible). In addition, on Yom Kippur the kohein gadol offers korbanot to atone for the whole people. What sins does that cover?
Perhaps this covers unknown sins, for which the transgressor never brought a korban because he didn't know he'd transgressed. (I remember learning this somewhere but don't know where. It also seems consistent with parts of the Yom Kippur liturgy today (as I've experienced it), where we confess for sins known and unknown.)
Perhaps this covers people who ought to have brought korbanot but didn't.
Or perhaps, even if everybody knew about all sins and brought all required korbanot, this is a necessary additional step -- just like a korban alone doesn't atone if you don't also make reparations, maybe reparations + that korban doesn't atone until you also have the communal offering on Yom Kippur?
So, what atonement do the Yom Kippur korbanot effect?