Normally firewall has a WAN ip that is public.. So if you do not use the built in firewall rule, or put in a wan address block.. Your block rfc1918 would not trigger..
So as always when wondering what you rules are going to do, walk down from top to bottom with your traffic and see what rule triggers or not.. If it gets to the end with no rules that allow or block triggering - then it would be blocked by the default deny that is not shown..
So lets say I took out that reject to firewall, and lets assume my firewall wan IP was 1.2.3.4, so now lets say someone want to hit the gui on my wan IP..
not triggered - its icmp and my dest is tcp 80 trying to hit gui
nope not dns
nope not ntp
not there
Nope dest is not rfc1918
yes my sorce is test net, and any is my dest port, and any is my dest IP.. Yup allowed, so now devices on this network could access pfsense webgui without that reject "this firewall rule"
If you wan IP is rfc1918, ie its behind a nat - then yeah you wouldn't need that rule. But you could use it as a safety net, on the chance you change your network and pfsense now has a public IP.