It's a long time since I thought about why the libunwind port is/was needed on 10.6, but here's one clue from many years ago about it.
https://github.com/macports/macports-ports/blob/676e6061b3a0150b4dff2ef682685092d71b8da4/devel/libunwind/files/0001-fix-_dyld_find_unwind_sections-for-pre-10.7.-Patch-b.patch It was part of Jeremy's project of bringing libc++ to SnowLeopard and Leopard, and the toolchain parts (cctools, ld64) that were involved with that.
