As far as I know, you use it to let the router work as a DHCP agent. forwarding all DHCP broadcasts to a DHCP server at another sub-net.
The following is what appears in ciscopedia:
ip helper-address address
no ip helper-address address
address Destination broadcast or host address to be used when forwarding UDP broadcasts. There can be more than one helper address per interface.
To have the Cisco IOS software forward User Datagram Protocol (UDP) broadcasts, including BOOTP, received on an interface, use the ip helper-address interface configuration command. To disable the forwarding of broadcast packets to specific addresses, use the no form of this command.
I try in any thread I start to not just get an answer for myself, but to make my thread a go-to source of information for people who are interested in learning more or trying to tackle a problem that involves what I'm discussing in the future.
That said, it appears we're off to a good start. Anyone else care to throw their hat in the ring with their own explanation? How about an explanation in more clear terms than Cisco offers.