1. You can buy ppv with out having a phone line attached.
2. Video on demand
3. If you run a Media Server like Tversity you can streaming movies etc from a pc to your box.
4. If you pay 30 bucks for Playon media server you can stream Hulu and Netflix on demand to your box.
I am sure there is more reasons but those are the ones I use it for except ppv as I buy that via Directv.com as the auth goes out to all the recievers and instead of just one .