Software developers from around the world will unanimously agree that writing code in test classes makes debugging more efficient. Why? That is because test classes help in creating robust and error-free code be it Apex or any other programming language. Since Unit tests are powerful in their own right, Salesforce requires you to write test classes in Apex code.
Why are they so powerful? Because test classes and test methods verify whether a particular piece of code is working properly or not. If that piece of code fails, then developers/ testers can accurately locate the test class having the faulty bug.
Test classes can be determined easily because every test class will be annotated with @isTest keyword. In fact, if we do not annotate a test class with @isTest, then it cannot be defined as a test class. Similarly, any method within a class which has the keyword testMethod, is a test method.