Campaign Groups in shopping ensures that a product is only advertised from a single product group and that this single product group will have its own ad group. This ad group will be unique across a campaign group.
This strategy ensures that your products never compete against each other. We also use our keyword based optimization based on campaign groups.
Ideally you should have only one campaign group for shopping campaigns. You should only create more than one campaign groups for shopping if you are
- Targeting more than one country. In this case, you should have one campaign group for each country
- Targeting different locations for all products. If you want duplicate campaigns one targeting some cities and other targeting rest.