Why does FTPS loadbalancing need 3 VIPs?

My Working Config

POOLS USED:

root@(bigip1)(cfg-sync Disconnected)(Active)(/Common)(tmos) list ltm pool ftp-port21-only 
ltm pool ftp-port21-only {
    members {
        10.1.20.100:ftp {
            address 10.1.20.100
            session monitor-enabled
            state down
        }
    }
    monitor gateway_icmp 
}

VIRTUALS SERVERS

The initial VS 1 which is required in the config to offload the SSL and no pool

root@(bigip1)(cfg-sync Disconnected)(Active)(/Common)(tmos) list ltm virtual SSL-FTP 
ltm virtual SSL-FTP {
    destination 10.128.10.102:ftp
    ip-protocol tcp
    mask 255.255.255.255
    profiles {
        ftp-ssl-profile {
            context clientside
        }
        ftp-tcp-profile { }
    }
    rules {
        ftp-ssl-irule
    }
    source 0.0.0.0/0
    translate-address enabled
    translate-port enabled
    vs-index 15
}

The Second VS needed on a different unused IP

root@(bigip1)(cfg-sync Disconnected)(Active)(/Common)(tmos) list ltm virtual ftpvs2 
ltm virtual ftpvs2 {
    destination 10.128.10.103:any
    ip-protocol tcp
    mask 255.255.255.255
    pool ftp-port21-only
    profiles {
        tcp { }
    }
    rules {
        ftpvs2
    }
    source 0.0.0.0/0
    translate-address enabled
    translate-port disabled
    vs-index 17
}

The third VS with the pool again.

root@(bigip1)(cfg-sync Disconnected)(Active)(/Common)(tmos) list ltm virtual FTP-all-ports 
ltm virtual FTP-all-ports {
    destination 10.128.10.102:any
    ip-protocol tcp
    mask 255.255.255.255
    pool ftp-port21-only
    profiles {
        ftp-ssl-profile {
            context clientside
        }
        tcp { }
    }
    rules {
        ftp-port-read
        ftpvs3
    }
    source 0.0.0.0/0
    translate-address enabled
    translate-port disabled
}
    vs-index 16