The main advantage of a compound fertilizer is that all the nutrients are contained in each granule or prill.  This means that there is no risk of segregation of the different nutrients during handling or loading as can often occur with blends.  Each prill or granule has a uniform size, shape and density which means they can be easily spread uniformly across the whole bout width, unlike blends where there is inevitably a compromise when it come to spreader settings.  The advantage of this a much more uniform and even distribution of all the nutrients across the field avoiding under or over fertilization leading to increases in both crop yield and quality.