Why am I required to use an IP address when Waking-on-LAN a computer?

A WoL magic packet can be sent either to UDP port 0, 7, or 9 (depending on the hardware in use) or as a raw Ethernet packet of type 0x0842. wolcmd has elected to use the former method, defaulting to port 7.

Note that wolcmd does support UDP broadcast, meaning that you can specify 255.255.255.255 as the address and mask if your hardware and network support TCP/IP broadcasts. The magic packet will only be interpreted by the machine whose MAC address it contains; all others will ignore it.