On parsing large binary file it's very inconvenient to load it completely in the memory for access complex structures.
In "Microsoft Compound File" internal FAT can be at any part of the file and sectors can be in any order like in FAT16(32) file system.
And if I want to load only the structure tree without data stream Then I want to get data from stream I must open and load it again.
This extension can be realized in any modern operating system and seems to be equivalent to (setf stream-element-type) - that is not realized in SBCL for example :-).
Yes the code using this will be unportable but it's Very Very convenient and intuitive.