Yield maps, if accurate, can serve as a good report card of what has occurred in your field that year. However, yield maps are different every year and even when using a 10-year average, they don't tell you why the yield was at a certain level in one area and different in another.
We know that if you isolate various areas of your field (create zones) based on soil electrical conductivity, topography, and water/nutrient movement you can build a reliable, consistent foundation (SWAT MAPS) from which meaningful soil samples can be taken each year. Therefore, using the information we need to make recommendations based on soil potential rather than past yield results.