From a little boy's point of view a father can teach him how to become a man and take on traditional male responsibilities. He can also prepare him for (competing) with other boys/men for key positions in life whether it be in sports or in life.
From a little girl's point of view her father can show her how she should be treated by men. He is the first man in her life. He sets the "standard" for which men she dates will be measured. He can also prepare her for how men think at various stages in their lives.
For example most teenage boys and young men in their 20s are not looking to settle down and "become their parents"! When they say, "I love you" it doesn't mean and shouldn't mean we're getting married. A father can teach his daughter how to tell when a man has his stuff together and is actually in a position to take on the responsibilities that come with marriage and raising a family. Very often it's age 30+.