you would need this if you have then VLAN entities as completely seperate networks, the vlan will stop any DHCP request going to a differet vlan. i.e. you have 2 rings each on different vlans so you would need a dhcp on each
alternatively you can use a layer 3 device and either route-on-a-stick or route on device so you can put dhcp on all the vlans and have it accessable to all units
dont really come across this in teh industrial world much so its normally a enterprise thing