The idea behind WiFi Priority is a simple. If you are in a place where multiple WiFi networks are accessible, your iPhone may sometimes automatically join the wrong one – and built in there’s built in way of creating a custom order for known networks.
This means you need to always go to the Settings to join the network you want. All WiFi priority does is let you select and sign into a network and then block it from auto-joining. (You can still connect manually via Settings, note.)
Although the UI design of the app could use an update (it is blown up on larger devices) – it does a good job.