What device are you talking about?
Typically, priority is used for fault tolerance as opposed to bonding or throughput. As long as the connection with the highest priority (lowest number is highest), traffic will continue to use that link. If the link fails a health check, that is when the link with priority 2 will be used for new connections.
This information is assuming that you are talking about outbound policies, but those aren’t visible in any of my dashboards, so I could be way off. If it is a SOHO or similar cellular based router/appliance - I don’t have any to play with. I could see how that might be useful for assigning WAN connection startup/availability based on other factors. I.e. prefer WiFi as WAN as 1st, then use simA if it isn’t available. I am pretty sure the mobile-y devices have such feature sets.
I am speaking from a Balance series router user perspective.