Additonal IPs on interface vs 1 to 1 NAT

Hi all 

Coming from a Cisco background I find these Sophos firewalls less than intuitive sometimes :) 

I need to setup NAT to an internal server on FTP and am doing it on an already preconfigured firewall. When I do NAT on other firewalls I usually just setup either a 1 to 1 NAT or a port forward and it works fine. 

When I look at this Sophos firewall there is a /28 subnet on the WAN side and multiple IPs within that subnet have been assigned to the WAN interface. Can someone explain why you would want to do this and the benefits of it as opposed to just creating a new host object and using that in your NAT rule ? 

Thanks