Campaign priority should be used when you advertise:
multiple products internationally in multiple campaigns
one product internationally in multiple campaigns
multiple products for the same country in multiple campaigns
one product for the same country in multiple campaigns
Campaign priority is useful when you’re advertising the same product, for the same country, in multiple Shopping campaigns.
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.
Read more here: https://support.google.com/adwords/answer/6275296