You can use a product group with a single bid to:
organize your ad group by product attributes
create different bid modifiers
ensure your entire inventory has the same bid
set a new campaign priority
You can use a product group with a single bid.
A way to organize your Google Merchant Center product inventory in a Shopping campaign within AdWords. You create product groups using attributes derived from your product data, available within AdWords. Each product can appear in only one biddable product group per ad group. You can also exclude a product from appearing in a product group entirely by using inventory filters.
Read more here: https://support.google.com/partners/answer/3517331