I'm no expert, but it has to do with loading programs. Natively compiled programs in OS4.x have the ELF format, and those are handled by this library.
I believe many of the changes made for this version were identified in connection with the work on the latest SDK and compiler versions, which to a great extent went on in public over at Amigans.net.
Elf.library is a very central component in OS4, and you should update it because it's likely to be required for future programs, if they are created with a recent version of the SDK and compiler - at least as I understood it.
Best regards,
Niels