If you use multimedia buttons more often that Function keys, you have the option to make dedicated multimedia buttons in Dell XPS 15z. Thus you can change the volume, play/pause, mute, change brightness etc in a single button use. If and when you want to use function keys as such; for example full screen in browser (F11), use Fn + the desired function key.
Control panel > Windows Mobility Center (or use Windows Key + X). Then change the 'Function Key Row' option drop down menu and change it to Multimedia key.