As you have already said - it's a ROM (Read Only Memory), it contains code to boot your computer up so it can access the network. This means you can have a computer without a hard drive, using the network file systems.
If it's an EPROM (rather than a ROM) you can erase it with UV light and reprogram it, you can easily tell - a UV eraseable EPROM will have a window in the chip for erasure.
If it's just a ROM, it's no use, except for booting your computer onto the network.