Possibly stupid iptables question.

I inherited an ipfire box and I'm looking at some rules that were previously created outside of the GUI within a startup script. I'm no iptables guru but I have made quite a few rules. What is the point of:

iptables -I FORWARD -d 172.20.0.0/16 -i green0 -o green0 -j ACCEPT

It looks like anything destined for that network will get forwarded out of the same interface it was received on. Why would you need that rule?

Is it possibly to force the reply to come out of that interface?

I admit this may be a rudimentary question for some, but I am tired and don't feel well so I'm looking for advice.