I think we’re seeing two use cases here.
@heiba probably opens Insync mostly just to check the feed. Then just a tray popup makes sense.
Some other users (like me) who use the selective sync/file explorer panel more often would definitely like a standalone window.
This is probably why many sync clients (e.g. official Google & Dropbox clients) show a popup to show feed upon first click, and then show a standalone window when you go into settings/preferences etc. This is way we can perhaps get the best of both worlds. Maybe Insync can consider a similar approach.