During the seven days of Sukkos, 98 lambs were brought (14 each of the seven days) to counter the 98 admonitions listed in Parshas Ki Savo (Rashi Bemidbar 29,18). However, an additional admonition is referenced in Devarim 28,61, which would not be listed.
Thus, the singular Korban of Shemini Atzeres is brought to counter the unnamed admonition and protect the people.
Thus, we can understand the verse in Koheles (11,2) “Give a portion to the seven, and even to the eight; for you do not know what evil shall be upon the earth.” The Medrash tells us that this hints to the seven days of Sukkos and the day of Shemini Atzeres. The end of the verse refers to the various levels of evils mentioned in Parshas Ki Savo, and the first part of the verse refers to the means to counter those evils.
Therefore, we have the 98 lambs from the seven days of Sukkos which counter the 98 admonitions (alluded in the verse as כל חלי, which is the gematria of 98) and the Korban of the eighth day counters the unnamed admonition of the verse.
(Abudraham: This verse from Koheles which alludes to Sukkos serves as one of the sources for the custom to read Koheles during Sukkos.)
