I think the first reason is for their own sake. Children thrive when they have boundaries. They need to know what is allowed and what is not. I think this doesn't change when we grow up, we feel more comfortable knowing what is acceptable and what is not. Try to imagine visiting a society that you do not know at all and which has very different customs from your own. How would you know how to behave? How would it make you feel to NOT know whether your behavior is appropriate or not? I think you'll agree with me that you'd feel much more comfortable knowing exactly what is appropriate to the locals. I believe it's not any different for children.
Child discipline is necessary to teach children how to behave in society. They need to learn how to behave at home, but they also need to learn what is appropriate in their own society. If parents don't discipline their children to teach them how to behave, they'll act like little anarchists. I, for one, do not think this is advisable and desirable.
When should we start to discipline our boys? We should start at birth! Discipline is not something that you can introduce at a certain stage and hope it will work. Many parents seem to first discover the need for discipline when their boys become toddlers. Of course, it's not too late to start then (it's never too late), but it would have been much better to start earlier.
Which "discipline method" to use is mostly up to your personality and your parenting style. There are a lot of different discipline styles, some better than others. There are two styles that I would like to discuss: