In reply to Game of Conkers:
I ran the Lakeland 100 last month which is 105 miles with a lot of up and down.
My training involved two long runs on consecutive days. Very early on in the training I was running 20 miles and then 14 miles or so the next day. At the end I was running 30 miles on Sunday and then 20 miles on Monday. This was 3 weeks out of 4.
I did all my training in shorts or standard running tights. I did use compression socks for recovery on occasion but found they made no significant difference. Recovery shakes (or milk) did make a big difference to recovery however. They made an instant difference to my recovery speed and I noticed it if I forgot to take one out with me. I also used recovery shakes during long runs.
On race day, I think I was the only one without any compression tights or shorts/socks. So anecdotely, for ultra distances, they work. But from my point of view, I made it round without any. Having said that, my thighs hurt like billyo and my feet were swollen to twice their size at the end. I think compression wear would have helped, but there's no way of being sure.
For injurys, I think they're useless. Much better to work out what's causing it and get a regular sports massage to control it. I had a sports massage every 3 weeks.
Er, so in conclusion, no idea but you certainly don't need them to do lots of running. Like most things in life, try them and see how they work for you.