for multiple reason:
- buying what you like. Pirating it is enough for a lot of people, but buying it is like giving the author what he deserve. supporting it by buying it.
- having an original can be considered an auto-satisfaction. it's like having a beautiful car that everyone enjoy or would like, but YOU have it.
- not opening it adds value to the product (well, maybe not now, but in future, if the disc becomes rare, like for Digicube goods). Old collection of Barbie dolls have a lot more value if the box is not open. It's the same here, except it's just that the product is new so it may not have a great value right now, but keeping it intact may help in the future.