1. To finally figure out the difference between the Guelphs and the Ghibellines. Dante was a Guelph.
2. To discover why Constantine made his famous donation.
3. To learn some new and ingenious ways to torture your enemies. Dante is very imaginative in this regard.
4. To find out what happened to Potiphar's wife, Mohammed, Ulysses, Atilla the Hun, Cleopatra, and Helen of Troy. We meet them all in The Inferno.
I recommend Dorothy Sayers' translation because of the excellent introduction and notes.