DHCP Discover broadcasts don't cross routers. Without help, you'd need a DHCP server on every subnet.
A DHCP relay agent (also called IP helper) solves this. The relay listens for DHCP broadcasts on its local subnet. When it hears one, it unicasts the request to a DHCP server on another subnet.
The relay adds its own IP to the message. The server uses this to determine which scope (address pool) to offer from. The server responds to the relay, which forwards it to the client.