I recently started making Chinese steamed dumplings (Jiaozi) with cabbage and beef mince. In this version I simply mix together all the ingredients for the filling (cabbage, onion, beef) when they're raw, fold the dumplings and steam them for a few minutes.
I've now seen recipes for vegetarian variants, e.g. with mushrooms, tofu and some other vegetables, plus the cabbage. For the mushrooms in particular some recipes suggest frying them in the pan before mixing them together with the rest of the filling. Is this always necessary and if not what are the pros and cons of cooked vs raw mushroom? Is it to drain some liquid first?