Been wearing them for a while now. Those can convert "complete ACL tears" (and out for the season) to "strained ACLs" (and out for 1-2 weeks - if at all).
Because of the amount of knee injuries to linemen by being rolled up on or stepped on this is definitely a preventive move. They help to stabilize the knee laterally more than from front to back. It does prevent all but it helps to reduce the amount of knee injuries.
As a former interior lineman on both sides of the ball (I wasn't any good we were just that bad) by the time you finish high school you have at least "tweaked" one knee in some way. So you had to wear a brace for support. And if it works on the bad knee then it outta help the good knee too right? Then once you get healthy and now you're used to wearing them why not just leave them on right?
The knees are the most exposed element of any interior lineman so they provide all the protection they can I think.
Back in the day, our high school QB club bought knee braces for every player on the team. Preventative, it didn't affect my speed at all, I was still slow.
I have a buddy who is a former offensive line man walk-on form the Dooley Era, and he says that they made all of the lineman wear the braces regardless of any injury or previous injury. The braces are very good at protecting the knees from lateral movement and hyper-extension.
They're necessary because most of the lineman are playing anywhere from 50to 75 pounds over their healthiest weight. Not many humans are truly able to carry 300+ lbs without damage to their joints.
Yes they all wear donjoys. I have one from when I tore my acl last year. They are super expensive. I couldn't imagine buying 2. They're almost impossible to tear your acl in them. If you tear your acl in one the company gives you your money back. That's how trustworthy they are.