Why is it a best practice to set a bid for the “everything else in all products” product group?

Q) Why is it a best practice to set a bid for the “everything else in all products” product group?

a) To be able to have one product group for mobile devices
b) To control the websites on which your Product Listing Ads appear
c) To make it easier to target a location for certain products

d) To include items not part of other product groups in your campaign