The SmugVault is for storage of virtually anything, especially files that are not normally supported by a regular SmugMug account.
One of the key features is the ability to bundle related files together by using the same filename when you upload them.
It is important to note that the SmugVault is for the owners eyes only. Visitors do not see the vaulted files. But, they can see rendered previews.
Details about the SmugVault can be found here: http://help.smugmug.com/customer/portal/articles/93324