You can use campaign priority to:
- Speed up how fast a campaign’s budget is spent throughout the day
- Have Google prefer certain inventory in query matching and ad serving
- Tell Google which campaign’s bid to use for a product when the same product is included in multiple campaigns
- Make your Product Listing Ads more competitive in the auction
You can use campaign priority to
Campaign Priority determines which products you want to get displayed from in case you have multiple Shopping campaigns running with similar products. In that case the deciding factor will not be bid, but the campaign priority if you set it to High.
When you have the same product in multiple Shopping campaigns, you can determine which campaign should participate in the auction for that product with campaign priority. Your campaigns already have a priority: Low. But you can change this priority to Medium or High. These priorities determine the bid for any product that the campaigns share.
Read more here: https://support.google.com/adwords/answer/6275296?hl=en