Why cant you just use a printer with out an ethernet connection?, beacause i want to connect my neptune 4 pro but just whit a usb-stick

“adding your printer” means you can start prints and monitor the printer via SimplyPrint; without connecting it, that part is hard to do. You can use SimplyPrint wirhout, and simply use our slicer, download the file and out it on USB.
I kinda get what you say though, and we are working on adding “Virtual printers”, so you have a printer “representation” in SimplyPrint, can manually log prints for it, assign filament and so on - but no live data, no “send prints wirelessly”. This will mainly be for show and for stats, but it doesn’t bring anything to the table print-wise that you can’t already do.