Levels are used to change the contrast of an image. It's especially useful for black & white images and pinpointing areas on the image that are 'true black' and 'true white,' and not to mention the mid-tones (which is all in between). In a sense, just like with people, every photo is different and has its own color scale even if all the images are black and white. That being said, its best to use a new Levels layer for each new image being worked on.
For a great tutorial on Levels, Cambridge In Color describes how to use the tool in a simple matter, so that even the most novice at Photoshop can utilize it correctly.
