"If you raise your child, you can spoil your grandchildren. If you spoil your child, you will have to raise your grandchildren." - Unknown
Discipline is a very important part of parenting. As a sinful species, no child needs to be taught to do wrong, but it is the job of every parent to teach the child to do right. Biblically speaking, this is called love. If you love your child, do you not teach them the right way to go? If they go astray, rest assured it will not end well for them. Thus it is loving to teach them to do good.
The problem with discipline is that it isn't something only a parent can do. Other people, be they individuals or agencies, and even nature itself, will teach an undisciplined child the lessons their parents never did. A child who is not taught to control their tongue will run their mouths at the wrong person. A child who is not taught to use diplomacy over violence will be thrown in prison. A child who is never told "no" will be told "no" when their pocket money runs dry. The world will teach a child the lessons their parents never did, and so parents must step into the role of teacher from day one. If you love your child, teach them the easy way.