What are the benefits of combining TestNG and selenium?

The TestNG is one of the most important component when you are actually starting and developing the framework from scratch level. Because you can have the control on all of your test cases and their execution with the help of TestNG.

So when I say you can have the control on your test cases there's a lot, so what all the important features we have in dealing and controlling test cases. I can give you some high level examples.

Suppose, if you want to run a test case "A", before that a prerequisite, you need to run some multiple test cases before you begin the test case "A". Now you can set and map the test cases in accordance to the requirement with the help of TestNG. In such a manner those prerequisite Test Cases will run first and only then it will trigger the test case "A", in that way you can control your test cases.

Now for example you have some 500 test cases and your client ask you to run only 40 test cases which are identified as smoke, now with one single click only the required 40 Test cases can be picked, out of those 500 and will be triggered and you will get needed results.

So like that you can control your whole test cases and how they have to run and what are their prerequisite / post-requisite and their execution categories so you can basically group them.

There are a lot you can do with the help of the TestNG in your framework.

You can also do parameterisation like you can run one test with multiple set of data with simple line by adding TestNG annotation. And you can also drive the data from external source(s). So there’s a alot one can accomplish with TestNG like-wise:

It gives the ability to produce HTML Reports of execution
Annotations made testers life easy
Test cases can be Grouped & Prioritized more easily
Parallel testing is possible
Generates Logs
Data Parameterisation is possible
It's also one of the main building component when you are dealing with frameworks.