Priority Group Activation, why use it?

Following is one of the use cases of Priority Group Activation.

A set of 4 servers has both web and ftp services running. The requirement is to have the first two servers serve web traffic and the last two servers serve ftp traffic, until any of them fails.

Ex. If the webservice fails on the first two servers, the web traffic gets distributed to the next priority group to meet the min-active-members condition, until the webservice resumes.

root@(bigipdc1ltm06a)(cfg-sync Disconnected)(Active)(/Common)(tmos) list ltm pool p_featuretest_80
ltm pool p_featuretest_80 {
    members {
        10.10.1.11:80 {
            address 10.10.1.11
            priority-group 90
        }
        10.10.1.12:80 {
            address 10.10.1.12
            priority-group 90
        }
        10.10.1.13:80 {
            address 10.10.1.13
            priority-group 80
        }
        10.10.1.14:80 {
            address 10.10.1.14
            priority-group 80
        }
    }
    min-active-members 2
}
root@(bigipdc1ltm06a)(cfg-sync Disconnected)(Active)(/Common)(tmos) list ltm pool p_featuretest_21
ltm pool p_featuretest_21 {
    members {
        10.10.1.11:21 {
            address 10.10.1.11
            priority-group 80
        }
        10.10.1.12:21 {
            address 10.10.1.12
            priority-group 80
        }
        10.10.1.13:21 {
            address 10.10.1.13
            priority-group 90
        }
        10.10.1.14:21 {
            address 10.10.1.14
            priority-group 90
        }
    }
    min-active-members 2
}
root@(bigipdc1ltm06a)(cfg-sync Disconnected)(Active)(/Common)(tmos)