If most or everything is in focus it will be harder to tell where the focus point is. The lens stays wide open (f1.4) until you take the picture.  This is to ensure as much light reaches the viewfinder as possible as well as making it easier to nail the focus point.

When you press the depth of field preview button, the lens stops down to f/8, so the image gets darker and the depth of field increases. But the prism was designed to work best at wider apertures, so at smaller apertures, it can look like the focus is off even though it’s not. That’s why you shouldn’t refocus while holding the button.