Medicinally
I make an infused coconut oil balm for aches and pains sometimes I mix in Meadowsweet with the comfrey for an additional boost. It is advised not to use comfrey internally
I personally limit use of the balm also as it is absorbed through the skin.
Primary use is as a fertilizer
comfrey has an extremely deep tap root that pulls minerals and nutrients from deep in the ground that are not readily available for most veggies. It has an excellent Carbon to Nitrogen balance so it wont hamper the nitrogen uptake of the veggies. Its also rich in calcium and potash. Better root development and flower set.
I use it two ways as fertilizer
dry and wet.
Dry
I harvest the leaves chop into smaller pieces and allow to dry. Then use a food blender to get it into a finer grain state and use as a soil amendment while rehabbing used soil and as a side dressing during the growing cycle.
Wet
the stems and leaves are roughly chopped an placed in a 5 gallon bucket with water and allowed to steep.
I use that water as a foliar feed and general spray on fertilizer like you would use miracle grow.
On edit
PLUS!
ITS FREE!