Johnny's is the closest place to the Oberlin campus that has a state liquor license. While all kinds of places (like Gibson's, the IGA) can sell liquor in Ohio, they're limited to 40 proof and lower, which frequently means that their wines are decent, and their rums are watered down. This can occasionally leave a person wanting a rum that has some kick to it, instead of just tasting like rubbing alcohol, and that's the void that Johnny's fills.
If you need alcohol on Sunday, some other stores in Oberlin don't sell it, so Johnny's can be the place to go. Johnny's also carries a wide range of fine microbrews that can't be found at other stores nearby. And they have relatively cheap packs of American Spirits.
Johnny's takes fake IDs and tapes them to the window to deter others from making the same mistake.