The principle behind the computer case, it to house all components directly linked to the motherboard in one space. Not only to make things look all neat and tidy, but to also provide protection to the sensitive components.
Another main reason for the case is so you can actually turn the computer on.
However, in recent years we have seen a change in computer cases, many companies are releasing 'pre-modded' cases, these cases usually now come with clear perspex side windows mounted onto the case pannel of the case, this enables users to see the internal parts of the computer.
It is not only companies that are changing cases, many people now take to modding their own cases at home as a hobby or professionally for a business.