More important here is that a VIP (for destination NAT) automatically does SNAT on reply traffic. Example:
you create a VIP mapping 184.108.40.206 (your WAN IP) to 192.168.14.4 (internal). The internal server answers and the VIP translates the source address back to the WAN IP 220.127.116.11.
Adding the NAT checkbox in the inbound policy would make the VIP use the internal interface address as source on inbound traffic, which would do no harm but would camouflage the original sender's IP address. All traffic to the internal server would appear to come from internal. Often, you prefer to know the external host's address for monitoring, statistics etc.
"Kernel panic: Aiee, killing interrupt handler!"