What you see is not what you get. When it comes to shooting with a SLR camera, this is usually true. The camera keeps the lens at its widest aperture for metering and focus operations. It only stops down the lens to the aperture you want when the shutter release is pressed to take the picture. This is typically desired because the viewfinder would otherwise be too dark and the auto-focus sensor would have trouble acquiring focus.
The problem is that the depth of field (DOF) is directly related to the lens aperture. How could the photographer know if the end result will have the subject captured in acceptable sharpness before taking the picture? To solve this problem, most SLR cameras are designed to have a depth of field preview button that allows the photographer to stop down the lens temporarily to preview the depth of field. Is depth of field preview the only function of this button?
It is definitely not. I came across this great list of things that the depth of field preview button can do by Rod Barbee. It is an eye-opening read, even though I have known some of these uses.
If you have given up on the DOF preview button in the past because it “just makes everything darker”, keep your eye on the viewfinder for a while so your eye can adjust to the darkness. Like entering a dim room from sunny outside, human eyes may take tens of seconds to become completely adjusted to the new light condition. The article also offers some other techniques that may help with this.