BV650 is the smaller of the two units and it is matched to what is actually plugged into it: the router, the display and the network gear, which together sit well inside 375W. Line-interactive with automatic voltage regulation means brownouts and spikes are corrected without draining the battery at all, which is the common case here rather than a full outage. Six IEC outlets means the network stack is covered without a power strip.
It stays plugged in permanently and does nothing visible unless the power fails. Its job is that a brownout does not take the router and the displays down at once.
650VA is not enough for a desktop and a monitor together, which is why the larger unit exists — splitting the load across the two is the whole arrangement. The stepped approximation to a sinewave output is fine for PC power supplies with their own filtering, but it will upset sensitive or older hardware. There is no USB communication port, so there is no automatic safe shutdown — if an outage outlasts the battery, everything downstream loses power anyway.