I do exactly this regularly, but it only works because for my most important folders (like personal documents), _somewhere_ I have a device online that is facil

I do exactly this regularly, but it only works because for my most important folders (like personal documents), _somewhere_ I have a device online that is facilitating the sync. In my case, my Desktop, Laptop, mobile phone (Android), and a Cloud Server that I use for other things. To answer the question most directly: If you have only your desktop and your laptop, then for the sync to work, at some point they both need to be powered on and online at the same time. This is one area where syncthing is a bit weaker than some other cloud-backed options; at that point you're essentially paying a service provider to replace the cloud server in my personal setup with their own always-on solution. Personally I prefer not to rely on a third-party, depending on your goals though you should pick a solution that makes sense for you.