In general, the purpose of fertilizing is to give your grass what it cannot get naturally from the soil in your yard.
In most cases, soils in residential neighborhoods are lacking in a lot of nutrients.
Sometimes this is because the soil itself is just poorly composed, to begin with, and other times, it may be because the lawn has been in place for decades and consumed everything that was there anyway, especially if you bag up your clippings and take them away each week.
This is where the idea of a soil test comes in - to show you what is in your soil.
Here is a video on how to take a soil test, step-by-step in your yard.
The double kits are great because you can test your lawn now, throw'er down and then re-test in the Fall to see how your lawn has progressed over the past few months!
Keywords: soil test, soil test kit, test, soil, test my soil