Why is this theory useful?
This theory avoids treating the characters as if they are individuals and reminds us that they are merely constructs. In other words some characters are there just to progress the narrative.
MY view:
This theory seems to be based in a male orientated environment as it is based on early folk tales.The theory is often dismissed by critics in regards to film. HOWEVER! it may still be applied because of the function (rather than the gender) of the characters is the basis of the theory
E.G. The hero could be a women; the reward could be a man.