You can use campaign priority to:

subdivide campaign products according to freely-assigned values with custom labels
edit maximum CPC bids, tracking templates, or custom parameters and destination URLs for non-subdivided product groups
promote certain product groups across all your campaigns
determine which campaign’s bid to use for a product when the same product is included in multiple campaigns

The correct answer is:

determine which campaign’s bid to use for a product when the same product is included in multiple campaigns.