It is for diskless boot. The ROM on the network chip knows how to look for a boot server from the local network (LAN), and load an OS kernel from that server. The kernel then loads the rest of the OS from the server.
There are two usual uses for it:
1. You can install an operating system into the local disk without floppy, CD, or other media. Requires a server that holds a copy of the install media.
2. Run entirely without disk, using remote network disks for all file storage. Quite handy when you have a cluster of hundreds of machines. But those rarely do use desktop boards.