The second definition to the word "propagate" tells you what this does: "spread and promote".
The propagate-settings clause takes data from where the DHCP lease was originally received, and uses it when clients query for DHCP on other interfaces / zones.
So the reason ge-0/0/0 is the default is that's where your ISP would connect (at least in Juniper's instructions but you can connect it wherever). Items you want to be propagated are done automatically, any overrides, you need to specify. So you'll want your network IP range/CIDR, "inside" router address, etc. to be different, but things like SIP settings, or DNS settings, Connection-specific DNS Suffix, etc. from your ISP will be propagates (spread and promoted) from the untrusted interface to the trusted interface(s).
Regards,
Ambi