Everyone hates testing(including me) as it's boring as well as you might end up writing more code for testing than the actual code which is under test. I learnt it the hard way by bringing down my team's application for a few hours when I tried to refactor code.

Ok, so why you need a framework like JUnit5 for unit testing?

A usual testing cycle will be like this,

public class Calculator {
       public int add(int a, int b) {
        return a + b;
    }

    public int subtract(int a, int b) {
        return a - b;
    }

    public int divide(int a, int b) {
        return a / b;
    }
}

If we want to test the above manually without any testing framework code will look similar to below,

public class CalculatorTest {
    public static void main(String[] args) {
       CalculatorTest test = new CalculatorTest();
       test.testAdd();
       test = new CalculatorTest();
       test.testSubtraction();
       test = new CalculatorTest();
       test.testExceptionInDiv();
    }

    void testAdd() {
        Calculator cal = new Calculator();
        int result = cal.add(1,2);
        if (result == 3) {
            System.out.println("Add test passed");
        } else {
            System.out.println("Add test failed. Expected 3 but was " + result);
        }
    }

    void testSubtraction() {
        int result = cal.subtract(2,1);
        if (result == 1) {
            System.out.println("Subtraction test passed");
        } else {
            System.out.println("Subtraction test failed. Expected 1 but was " + result);
        }
    }

    void testExceptionInDiv() {
        try {
           cal.divide(2,0);
           throw new Exception("Fail");
        } catch (ArithmeticException e) {
            System.out.println("Divsion by 0 test passed");
        } catch (Exception e1) {
            System.out.println("Divsion by 0 test failed");
        }
    }
}

For each condition, we need to write a conditional block and also make sure the error message is readable. This way of testing also results in special handling for cases like what if one wants to skip a few tests or skip a test if a system property is missing(@EnabledIfSystemProperty annotation in JUnit5) etc. This is where a testing framework like JUnit5 helps.

So the testing cycle will be reduced to this,

I will end with this, a testing framework helps us in writing better tests.