Why restrain horses?

Answer 1

Restraints are used as a substitute for training, and force the horse into something, rather than teaching it. Tie downs are used to force the head down, rather than teach the horse to hold his head down. Twitches are a force, used to distract a horse so he will hold still while the farrier or whatever is doing something to him, instead of training him that the person will not hurt him, and teaching him to hold still on command. Hobbles also force the horse to hold still rather than training.

In my opinion, any good horseperson will not use restraints, but rather teach the horse. Restraints may be used in an emergency, when you need the horse to do something, and have no time to teach him, but he shoudl be trained after the emergency has passed, so that next time, you will not have to use a restraint.

Answer 2

Simply because they will run away. Do you mean with a rope or in pens/fields/stables? Horses are not always completely intelligent and when loose, they will easily run on to a road. I've known horses to escape from fields and before you know it, they're miles away. Some horses don't need restrained as much as others. Some you have to keep a close eye on and others you can walk to the field and it'll happily walk beside you without a rope, etc.