In addition to the "why does LLVM matter" asked elsewhere, why would I want to use OpenMandriva?
but I'm not really sure. I guess if you want a recent kernel, KDE Plasma based distro and you work with LLVM/clang.
I really don't know where it fits with Mageia/PCLinuxOS and other Mandrake descendants.
History IIRC -- Mandrake was RH Linux with KDE; Mandriva was a continuation of that which split; OpenMandriva were devs from that split that took ROSA Linux (still doing KDE4 I think) and then continued their project from that base.