As I understand it, comfrey usually only spreads if you dig around the plants, breaking, and spreading little root bits that sprout into new plants. I've had 2 comfrey plants for almost 10 years. Never a new one popping up. I planted them in a permanent spot and purposely have not moved them or dug around them.

FataMorgana

Mine slowing sprouted babies/off shoots. I never had any from seeds.

I have Bocking14 which is a variety of Russian Comfrey, Symphytum upplandicum, rather than Common Comfrey, Symphytum officinale. The patch has spread slowly over the years but is not a big problem. Pieces will root if spread about but are easy to pull. You are rightthat it is a sterile cultivar and does not set seed. (Bocking is a village in Essex, England, where the strain was developed)

They grow wildly on my property. We've never planted it ourselves, so it could have come here from birds pooing the seeds!

What do people actually do with it? There are big patches in different spots right now.

I cut mine for the compost heap and to use as a mulch. You can also make a 'tea' out of it.

Here is a link that might be useful: What to do with comfrey

I know people who like to make external-use salves with it.

FataMorgana

I'd better clarify - when I said 'tea' I meant compost tea. Comfrey is strongly discouraged for internal use ie don't eat or drink it yourself. It's for the garden except as Fata says. Its traditional name was 'knitbone'.

I purchase a homeopathic jelly that has comfrey in it, designed to relieve arthritis.

I also rub it on if I strain or bang an ankle or wrist, and it seems to speed healing.

I did steam and eat the leaves at one time, before all the hoo-ha about harmful this or thats in comfrey. It was delicious--tasted a bit like asparagus (leading me to idly wonder what "compounds" asparagus might contain).

I used to make a flat pile of the young leaves, bend the pile in half, and slice thin, then place in the steamer. I only did that in early Spring, before other greens were available.

Although comfrey forms bunches rather than spreading, it has a little survival-competition trick that is not too friendly to nearby neighbors. It will grow straight up to about two feet then flop over, essentially shading out anything next to it to a 2 ft diameter or so, like an umbrella.

On the other hand, it is simple to divide the mature bunches to get more comfrey plants to plant elsewhere. Spring would probably be the best time for that.