No it is not an interview question! I was going through the incremental load that time I got to know about the buffer load that could not understand from the help file. If you mean to say that is used to create and maintain the QVD. Can I assume that it is same as STORE command? If both are not same then what would be the differences?
Buffer- If the keyword is prefixed before the load QVD files can be created automatically and stored in the Application Data folder. You can check the user preferences- Folder for the location details.
There are limitations which you check in help.And the QVD will get deleted automatically when the document you have created is deleted.But you can restirct it getting deleted automatically by enabling by Keep orphaned Buffers.
I would suggest to go through Help option which will more clarity.