Convenience and portability – take it anywhere without worrying about breakages, and no corkscrew needed.
Freshness – open it up and it will stay fresh for much longer than an open bottle.
Price – most three-liter box wines sell for $16-20. That's $4-5 per 750ml bottle, which is considerably cheaper than the average price of around $9.40 per 750ml bottle.
Environment – less packaging waste and less energy needed to transport.