Q. Why do we say Al Chet by Mincha on Erev Yom Kippur?
A. 1) Because someone might choke on his food at the Seuda Hamafsekes and not get a chance to daven again.
2) Because someone might get drunk at the Seuda Hamafsekes and be hung over all Yom Kippur!! (Yup, the Rambam actually lists this reason!)