With any slow or quick release fertilisers, granules will begin releasing some nutrients immediately upon application and/or watering. Granules that continue to sit against the trunk/main stem can continue to release all it nutrients on it directly and cause some superficial burning. This can cause damage to young plants, but rarely on mature plants and trees. Watering in the product immediately upon application will help avoid this problem, should the granules get on the stem
Another reason is that the nutrient seeking roots of most plants are located away from its trunk and towards its dripline (the line of the outer leaves of the plants). So, it is beneficial to feed where the plants roots can take the nutrients in i.e.- less waste.