In a nutshell, spot metering enables us to select different exposure points ( depending on your camera body ! ) mine allows me to select up to 8 seperate exposure points per shot - and in the right situation it really is a godsend.
So if you have a bright sunny day but have shadows, you can select the parts of the image which are darker and lighter and over / under / correctly expose them accordingly in the same image. You'll only really need to use spot metering for static objects as it's a more time consuming process than normal photography since you'll be using the camera to sometimes several different reference points for exposure every single time, so for action shooting spot metering is a no-no ! If you see someone with a shadow on their face you can re-position them OR you can take spot readings of the darker part of their face due to the shadows and the camera will deliberately overexpose this section of the photograph to bring the the brightness up to the same level as the parts of the face not in the shadows. Or a scene where you have a bright sky and a darker ground, or sunlight reflecting off water - make seperate exposure points on elements in your photo which are noticably brighter or darker to compensate.