Comfrey is a leafy plant with incredibly long roots that draw up minerals and nutrients from deeper in the ground than your fertilizer applications can penetrate. The leaves are loaded with nutrients which can be harvested and used for feeding your soil and plants. The NPK ratio [See: Understanding Fertilizer Labels] of comfrey leaves is 1.8/0.5/5.3 which is significantly higher than home made compost and other organic fertilizers. NPK is nitrogen, phosphorous and potassium but comfrey is also loaded with calcium and trace mineral that plants need and lap up readily.
Once comfrey is established, dig leaves into the soil at the base of vegetable plants to avoid having them blow away, just a 1 to 2 inch layer of soil atop will suffice. Over time they break down and nourish other plants.
You can also make a liquid fertilizer out of comfrey leaves by mashing the leaves and steeping in water for a few days, basically a comfrey tea. The enriched water makes an awesome fertilizer for fruit bearing plants such as tomatoes, peppers, berries,cucumbers and so forth. It's not all that great for leafy vegetables such as lettuce or cabbage. At seasons end comfrey leaves still remaining can be composted.
A suggestion for young perennials such as asparagus, fruit and berry trees and bushes and so forth. At planting time bury some comfrey leaves in the planting holes. As the comfrey leaves break down, they will nourish the young plants.
Comfrey also has some anti fungal properties, I wouldn't advise it as a fungicide but it has been demonstrated that it prevents powdery mildew spores from germinating. 'Scientists at Moscow State University discovered that comfrey tea, applied as a foliar spray, slowed the growth of powdery mildew spores on plant leaves [Bobby Quinn, Kerr Center]'