The four functions of law include defending people from evil, promoting the common good, resolving disputes and encouraging people to do the right thing. These four functions are essential to human welfare.
The four functions of law provide a basis for how society would optimally function. Defending people from evil declares that the law will protect humans from other people who want to do harm to mankind. Promoting the common good essentially provides support to those who organize themselves into a group to fight for specific rights.
In the case of resolving disputes, it provides support to people who cannot agree with another person, such as during a divorce. Encouraging people to do the right thing came about during Aristotle's time, but it still remains true today. Having a law system helps people feel more comfortable about reporting injustices and crimes that they may witness.