"One Picture's Worth Thousands Words"
Benefits( from my personal experience):
1. Development can be done much quicker. You already jot down all the test cases before writing codes. By that way, you are giving much time in thinking how to solve problem. Codes become much more standard and organized because of the brainstorming.
2. Quality of code is far better and deterministic.
3. Tests are the first class document for how your code will eventually work. Any developer/tester can go through the test cases you covered and get the idea how the system should work in different cases.
J Isn’t it too cool ?
4. Easier and risk free to change the code - you just need to run the tests, you will get to know where there might be some problem.
And so on...