IpTables

Hello I have been trying to start to learn about IpTables. I have a general understanding of the tables, chains and some commands. But what I can’t understand is the benefits of dropping packets in the Prerouting phase vs the INPUT phase. I looked at the flow chart and I see how it works. But I still can’t see why it would be better to drop/accept packets in on phase or the other. For example whats going to be the difference between these two commands? They seem like they would do the same thing: iptables -I PREROUTING -t mangle -d 198.18.0.12 -p udp --dport 1234 -j ACCEPT

iptables -I INPUT -t filter -d 198.18.0.12 -p udp --dport 1234 -j ACCEPT