In DISCO we create CAL modules that are purely for caulations and turn Summary Method to NONE.
There are though many times where to be most efficient you may want to only have the Summary method apply to one list.
e.g. you want it to SUM on product only because you can then use it in an Output module which doesn't need the data by Product.
being able to select the Summary Method per list will allow for more efficient and faster models.