Let's consider the following scenario:
There is already a small LAN with static IP addresses everywhere. You want to make administration easier by pushing various settings over dhcp (gateways, ntp servers, dns etc) but you don't want to change the way existing connections work, so you want to keep the same IP addressing in place.
To achieve this in a manually fashioned way you would install a dhcp service on a dedicated machine, perform an arp scan, collect all arp entries, then with a little sed and/or awk generate dhcpd.conf fixed-address host declarations inside the dhcp range declaration. However, since we are lazy people, we would like to have a push button to do all this magic for us, so all that is left for us to do is to fill out the host name for the discovered pairs of mac:ip
It would be awesome if this eventually becomes reality under Zentyal.