The Five Evils

דברים לב ,כד: מְזֵׁי רָ עָב ו לְחֻמֵׁי רֶ ש ֶף וְקֶטֶב מְרִּ ירִּ י וְש ֶן-חַ לַ שֲא תֹמֵׁהְ ב-ב ָם עִּם-חֲמַת זֹחֲלֵׁי עָפָר:

Baal HaTurim states that the five forms of retribution mentioned in this Pasuk (bloating of hunger, burning heat, suffocation by acrid air, teeth of beasts, and venom of ground creepers) corresponds to the five Books of the Torah that they contravened: the five immoral acts listed by Hoshea, the five abominations that Yechezkel saw, and corresponding to five false deities they constantly worshipped (Ba'al, the sun, the moon, the Mazalos and all the hosts of the heavens).

Perhaps this also explains the need for the five Tefilos on Yom Kippur.