It’s used for out of band management. With the correct hardware items (nic and gpu) it’s called vPro. With the proper certificate and supporting infrastructure it can auto-enroll into a management service such as SCCM. It allows companies to remotely view logs, bios settings and other items. With vPro it can include a complete remote KVM solution.
You can disable it from most UEFI settings interfaces without worry of causing other issues.
I manage storage systems as part of my day job. i think you would be happy with a simple direct attached storage device. You’d need a storage controller card and a storage controller. These are usually enterprise-grade items so they might be expensive. I suspect there are SATA options but SATA is pretty slow.
QNAP and Synology are decent for what they offer, if you like the idea if turning it on, setting up an account, and then having access to both native and an easy 3rd-party store with no fiddling needed then they are a good idea. You can also setup an iSCSI connection for direct-attached storage over the network.