There is no software involved – the SSR is operated by the PC's USB 5 Volt supply.
This one was used when my home network started to be attacked (in the early hours of the morning). I decided, rather than leave it running 24/7, I would send it to sleep from midnight till 6am. Although the NAS would go into standby when the server went to sleep the cooling fans continued running. This device switches off the NAS completely during the server sleep period.
Another is used to switch my UC400ETH external motion controller PSU on and off with the CNC machine PC.
And as said I just keep finding uses.
Tweakie.