What is the biggest advantage using junit? [on hold]

When I have needed to test some java class, I have been made Tester.java like below.

public class Tester
{
    public static void main(String[] args)
    {
        Sample sample = new Sample();
        sample.sayHello();
    }
}

Is this diffcult? Absolutely no. Isn't it? It is very simple. and enough to test any java programs.

I don't need to use assert series method of junit. To see test result, do I have to use junit framework, junit's assert methods? I can see test result on CLI console. printed by "system.out.println" It is not huge chunk. that is just 7 or 10 line. that' all.

I don't know why many engineer show their enthusiasm with junit?

The code which is nesessary for test program is just like that I described above. just 7 line. Is is hard?

Why do you use junit?

I have searched on internet why? but there is only "how to use". but I want to know "why use"

Anyone please answer me. Thank you.