1. It increases arousal, which would hopefully increase sexual desire.
2. People use it as more of a "complete solution" / substitution.
3. People use it as a tool to avoid confronting and dealing with the inevitable problems and complexities which are bound to arise in any long-term relationship.
To me, pornography seems like such a poor substitute for real life connection and experiences, I've found it doesn't replace anything beyond, uh, basic "maintenance" needs. Porn is enjoyable, but pales in comparison to a real interaction with a real live person.
Between this and the depressing macro-economic climate, our generation has a tough road ahead.