Simple Answer: A properly tuned enclosure allows the subwoofer to perform its best. 
Yes, it can make bass from just any box, but unless it is tuned for your specific subwoofer(s), it is only going to sound good on a small range of frequencies.   If an enclosure is far enough out of tune, it can also damage your subwoofer(s).  People who run out of tune boxes typically have to find that one song that sounds good and are limited to how many songs they can listen to.  If you are going to invest the time and money into an expensive car audio system, use a tuned enclosure to ensure optimal performance from your subwoofer(s).