Arping can be used to find out if a specific IP address on the LAN
is 'taken' and what MAC address owns it.  Sure, you *could* just
use 'ping' to find out if it's taken and even if the computer blocks
ping (and everything else) you still get an entry in your ARP cache.
But what if you aren't on a routable net?  Or the host blocks ping
(all ICMP even)?  Then you're out of luck.  Or you use arping.