depends_lib-append question

I think this topic has been covered but I can't find it in the archives.
What is the purpose of using depends_lib-appends by itself?  I see some
ports use this with no variants and I don't understand why.

Also, someone has submitted a patch to the Mozart port because "the lib
dependencies get overwritten" that does this:
 
+depends_build	     port:gmp port:zlib port:gdbm
 depends_lib-append   port:gmp port:zlib port:gdbm
 depends_lib          path:${prefix}/bin/emacs:emacs

Can someone explain why that might be necessary?

Mark