Comfrey is a perennial flower. I grow it in my garden as a flower. It grows vigourously and comes up year after year.
The main value of comfrey as a fertiliser is that the leaves are high in nitrogen.
You can dig the plant directly into the soil if you want to. Or you could keep some a a living plant, to allow you to harvest the leaves. These can be left to decompose in water which will make an excellent liquid plant feed.