Priority Group Activation, why use it?

Another example for priority groups usage in real world

2 varnish servers          priority 30
5 main servers             priority 20
2 static content servers   priority 10
1 maintenance server       priority 1

Priority Group Activation  less than 1

So in this example you have 2 varnish servers taking care of all requests, but if both die, BigIP will start to ask the main servers directly. If they also die, static content server are ready as a emergency server, with a limited version of the site. if those also die, a server that will report a static maintenance page on all requests will be the last one serving the requests.

So this way you try as hard as you can to maintain the site working using the priority groups.

Other example is to isolate sites on share hardware... one site have 5 servers and a different site have other 5 different servers. you can setup 2 pool with all servers, with 50% of the servers with higher priority, but each pool using different high priority servers. in normal work, each site will use their own servers and will not affect the other. Using "Priority Group Activation less than 3", if 3 servers die, BigIP will start using the other site resources to try to maintain the site up and will free then up if the main servers are back online. So you can have dedicated servers that are shared in emergencies. By removing other site servers from the pool, you can even enforce that you only allow some shared servers and keep certain level of dedicated servers