It's not a cheap test, and therefore may not be that useful for a beginning gardener. This would be helpful if you're trying to maximize output, for example if you're selling your produce and need to make as much profit as possible. You send in soil samples to a qualified lab, and they determine the type and number of organisms present. The lab will also want to know what you're growing. You'll find out what types of creatures you have and what types you should have.
Different organisms improve yields for different fruits, vegetables and grains.
Then by adding microorganisms or changing your gardening techniques you can encourage the right types of microorganism to flourish. The last type of soil testing I'm going to discuss is pesticide testing. Right off the bat, I'll tell you that unless you have a specific reason for looking for a specific pesticide, you probably won't want to do this. First, if you look hard enough you will find pesticides everywhere. They come down in the rain, they're carried on the wind and swept along in the water.