Finding information on this card was very hard, and here is the best that I could find:

With front and back LEDs and a removable system configuration card, the Sun Fire V120 server maximizes system availability by allowing system administrators to concentrate on scheduled service through easy installation and management. The removable system configuration card allows you to store a system’s host ID, MAC address, and NVRAM settings to another server while you perform routine maintenance. As a result, system downtime is minimized.

Why I find this interesting is that it is also a nod to commercial software companies relying on hostids for licensing. In this way, you can maintain the same hostid even when a server has issues, and without compromising the integrity of licensing. Sun’s hostids are unusually safe and reliable, unlike the common x86 anchors like Ethernet MAC addresses (which are easy to change) and disk IDs (which are not available on Linux typically).

Making the ID physical in this way is usually the best way to handle identity in general. A GSM/UMTS SIM card is another example of a physically represented identity, which is way preferable to virtual identies that are just software. Much easier to handle, and safer for all involved.