If configured in the workspace settings, this API server is accessed from within LibrePCB for the following reasons:
When opening the library manager, the list of available libraries is fetched from this server and displayed in the library manager.
The "Order PCB" dialog in the schematic- and board editors allows to upload a LibrePCB project to this server. Afterwards the order process can be continued in a web browser. When continuing the order process, the project will be forwarded to the chosen PCB manufacturer. For details and privacy statement, check out fab.librepcb.org/about.