When I teach Lightroom or Photoshop, I make a big deal about setting blacks and checking whites. In fact, that is something I always do with every photo. I will never allow anyone to use one of my images from a RAW file without doing this. A JPEG file from camera is a processed file, so in a pinch, I will send a JPEG file when I have to quickly (which is one reason I shoot RAW + JPEG).
This is actually not a Lightroom, Photoshop or computer thing. This is about how photographs display and goes back to traditional black-and-white photographers like Ansel Adams. Adams talks quite a bit in his books such as The Print (which is still in print and a superb book for anyone processing images, even in the computer). He discusses how important it is to have a full range of tonality from black to white or a print will never look right. It will be gray and look much less attractive because it does not fully use the tonality available in the print. It then looks duller than it should. Adams talks about how important it is that most scenes and subjects need that full range, something a pure black and something a pure white, in order to look their best, though some scenes, such as a foggy day, will not and should not have that range.
Pronghorn in front of Large Array Radio Telescopes in New Mexico; first without proper blacks and whites (from camera), second with blacks set and whites checked:
In today's digital world, this is still important. There is a range of tonality and color available for any display, whether that is a print or an image on a monitor. If the image is not using that full range of display capability, it will never look its best both in tonality and color.
A RAW file is not set up to give you the best range of tonality and color from black to white. It is designed to be processed. In fact, most manufacturers automatically elevate the blacks to gray because a weakness of any digital camera and its sensor is the dark areas (this has to do with the physics of how a sensor works). If you don't then set blacks and check whites (as well as control mid tones), you are not getting the best from the original RAW file.
Notice that the contrast and color looks better simply by working with blacks and whites. Setting blacks and checking whites ensures that your image is using the full range of tonality and color. Blacks are more subjective and you often have a range of possibility for them. Whites tend to be very sensitive and you need to be careful about them. That is why I lke to use the terms setting for blacks and checking for whites.
All Adobe products make this very easy to do. You press the Alt or Option key as you set blacks and whites and the screen will show a threshold screen displaying exactly when and where blacks and whites occur in your image. This is the first thing I always do with an image. In Lightroom and Camera Raw, you use the Blacks slider in Basic for setting blacks and the Whites slider in Basic for checking whites. In Photoshop and Photoshop Elements, you use Levels and the black slider at the left for blacks, the white slider at the right for whites. And at the risk of being repetitive, you press Alt or Option as you do this so you get a threshold screen to look at. This next image shows the blacks threshold screen in action.
Blacks will show up as pure black on the threshold screen. For some images, this may be small spots of black, for others, large areas of black. It really depends on the photo. If your image is full of color, you might not get a pure black, but you will get spots of color. Whites show up as pure white on the whites threshold screen. I usually just barely have a spot of white or color show up, and sometimes, I back off the adjustment slightly until the whites just disappear. Here is the image that was being adjusted in the Lightroom screen shot, before blacks and whites, and after blacks and whites have been adjusted. No other adjustments were made to this image.
The image may be too dark or too light at this point. This is when you adjust midtones to bring out detail there. I will often tweak dark and light areas with the Shadows and Highlights controls. Use Exposure and/or the Tone Curve in Lightroom/Camera Raw, or the middle slider in Levels or use Curves in Photoshop products. Especially watch for muddy, murky dark tones that digital cameras often struggle with – Shadows in Basic as well as Darks in Tone Curve work really well for this with Lightroom/Camera Raw.
All of the images here are from New Mexico shot this spring. Also, every image in all of my books, including my new photo e-book, Reports from the Wild, have blacks set and whites checked (though foggy or hazy days do not have blacks or pure whites).