When you mount a DMG file for example when installing an app, the default style of Finder window displayed is one without the sidebar and toolbar as shown above. You can easily show the sidebar and toolbar by clicking on the button on the top right corner of the Finder window.
You can hide or display the sidebar or toolbar individually from the Finder View menu. The inconspicuous top-right button allows you to hide or show both sidebar and toolbar together with a click. Hiding both sidebar and toolbar makes the Finder window much cleaner to work with.