Some file formats have binary blobs that are not big lengths. There are a handful of ways to model these as is (e.g. xs:integer, two fields where one is a byte length hexBinary blob and the other is the remaining fragments), but they are inefficient and clumsy. We really want a way to pull out necessary bits and treat them as a binary blob.
Note that the DFDL specification does not currently allow bit length hexBinary, so this must be implemented as an extension controlled by a tunable.