When you launch the Quickbuild command, a viewer of some sort will normally be called after the compilation. If you are not using an embedded viewer, a new window will be opened every time.

If you are adjusting the look of your document, you might launch Quickbuild very often, and have many viewer windows open on your desktop; to avoid this confusion, you can activate the Watch file mode, that will prevent Quickbuild from launching a viewer.

This mode is of course useless with the embedded viewers, as you have to close them anyway to get back to editing the document and recompiling.