In TestNG, we cannot define a suite in the testing source code, but it is represented by one XML file, as suite is the feature of execution.
testng.xml is an XML file that describes configuration of TestNG test suite. Using Testng.xml file we can run test available in one or more class file and make them as a single group, hence making test more meaningful, we call them as scenario-based testing.
Can we run the test without using testng.xml file?
Well, the answer is yes, you can run. But once your test become larger and requires a configuration of multiple tests at one place, the best option is testng.xml.