A reader who chooses Online on the overview often expects the detail page to keep that choice. Sync slicers shares the selection across chosen pages. The pane has separate Sync and Visible controls, so a slicer can affect a page even when it is hidden there.
That separation is useful and easy to misuse. A hidden channel filter can explain why a detail table seems to have missing rows. Make the active context clear somewhere on the page. Turn Sync off when a page needs an independent comparison, rather than deleting its visual copy. Test the destination after changing the source selection, and check that a hidden copy still filters only the pages you intended. A normal page filter doesn't become synchronized just because it uses the same field.