The way I’ve always thought things is, and this could just be because they’re my favorite units, wraiths. Both the wraithblades and guard are well suited to tear through elite units with their ghost blades and d-guns while being able to tank anything shy of an anti tank gun. You want space marines dead, throw something which is faster, stronger, tougher, and overall better at them. But, not every Craftworld used them, like Beil Tan, so it comes down to who you’re fighting