These would typically be used by an ISP to hand off a dedicated internet access connection or an MPLS or VPLS circuit. At the core, it's an Ethernet switch, but it also supports supports carrier Ethernet features like G.8032 for ring topologies, RFC2544 for throughput measurement (a sort of built-in speed test that works between two compliant devices over a layer 2 link), and Y.1731 for continuous latency and jitter measurement.
According to the data sheet, it also supports port mirroring, so it should work to put one of these between your ONT and firewall to mirror your WAN traffic, although it's probably overkill for that application unless you've stumbled onto a used one for a great price.