Without this, appstreamcli (used by discover and dolphin) would rely on LD_LIBRARY_PATH to find the correct libappstream.so, and that's not great because the standard prefix.sh doesn't set LD_LIBRARY_PATH, relying on rpath instead.
Appstream's meson build doesn't set rpath for installed binaries by default, but can be prodded into adding it with LDFLAGS.