There are several software vendors still using older 2.x versions of OpenVDB libraries, for whatever reason, whereas most are using OpenVDB 3.x (4.0 was released a few months ago). That flag switches the version of OpenvDB being used write files back to OpenvDB 2.0/2.2, which is what H13 was using. A bit more background about why you have to write out files again after setting HOUDINI13_VOLUME_COMPATIBILITY, for anyone curious.
