They need a small back story and a purpose. Where did they come from and where are they going? Why are they where they are? Why would they feel the need to fight with adventurers or why are they in a position for them to be fought. What is worth them dying over?
Don't have to answer all the questions, or even any of them if it is a simpler purpose. But need to answer why and then get that information to the PCs somehow.