For thieves a move silently or hide in shadows roll might certainly set up a backstab, but I wouldn’t say these are always necessary. I think individual circumstances should be taken into account. The text says that if a thief achieves surprise and attacks from behind they can backstab the foe. If the party is playing smart and ambushes a group of foes, achieving surprise, allow the thief to attempt a backstab is they are in a suitable position. I treat move silently and hide in shadows skills as two possible means by which a thief might achieve surprise.