Why Manage Site Specifically?
Each field is variable. This variability comes in the form of soil types and texture, water holding capacity, topography, nutrient availability and historical management. Your fields aren't uniform, so you shouldn't treat them that way.
By managing fields site specifically, you have the opportunity to target high yielding areas of your fields and be aggressive with management strategies to increase yields in those locations.
While you are able to push inputs in yields in some portions of the field, you also have the ability to back off on inputs in portions of the field that typically don't yield as well or are subject to more variable conditions
By increasing management in high yielding zones, and backing off inputs in low yielding areas, you will work towards increasing your ROI by channeling inputs to the exact locations in the field that need it, and can capitalize on it most. High yields aren't of much value to a producer unless they are making you money.