If I remember correctly, it's to get the buff from UE applied to both LvB and FS; FS will hit the mob before LvB due to LvB's travel time.
If you open with UE>LvB>FS your LvB wont get the +50% from FS. So it's not worth it. The game checks for FS at cast rather then hit. That also means if you finish an LvB cast right before FS ticks out it will still get the +50% dmg increase.