photoshop | when to use the blur tool - CORRECTED
The blur tool is on your photoshop toolbar. I think its quite useful for softening a photo that may look too pixilated. A lot of times when you start adjusting the levels to brighten faces, the pixilation of the photo also appears. Here is an image that I caught of Noah napping. You can't really tell, but he had pulled off his socks and shirt and was sound asleep. I took this picture and he didn't even wake (that is until his sister came in yelling "surprise, are you so happy buddy?"). The sheet in this photo had gotten a ton of fuzz balls from his blanket & then when I adjusted the levels it showed even more. The first is without the blur tool applied, the second is with the blur tool applied.
This is not a fix for an extremely underexposed image. To do that, you'd have to create a couple duplicates of that image and layer. But for fixing a slightly grainy photo, this is the cure. A lot of times, if you are displaying your images online, this isn't even necessary. However, if it is something you've adjusted the levels for and want to print and frame, I would definitely recommend this. It gives a much cleaner finish.