You have to think of a horse as a car with the engine in the back, all the power comes from the hind end, if the horse has enough muscle in the hind end and stomach they can hold thier head up the side riens are just a guide and enforce the position the rider wants the horses head in
That is false, if you use them
Correctly they teach the horse to carry their own head and help them gain muscle so you don't work as hard. I absolutely love them for my stubbornn 17 hand tb it has made him so much lighter in my hands and he has gained so much muscle
It depends on how you use them. I've seen many many people tie them too tight so their horses heads are btv and it's awful to watch. But with correct usage, they can help. They should not be too short so that the horse's head is fixed and restricted, but not too loose so that it can raise its head and become hollow. You want your horse to seek the contact :)
It helps with learning and with those horses that are harder to put on bit.