The tapioca is an alternative source of starch, which is required for manufacturing kibble, or it will just fall apart to crumbles. In most grain free products (Evo etc.), potato or sweet potato is used for this purpose, in traditional kibbles it's grains like rice, barley and so on. These are not "fillers" though, but actually sources of highly digestible carbohydrates.
Pumpkin seeds contribute fatty acids, some trace minerals and vitamin K.