Another benefit is re-usability. If you want your program to be callable as module or script from elsewhere without going through the tedious shell functions, "def main()" is the only way to go.
The difference is that you've encapsulated your coding logic in mainMethod() instead of writing under that if condition itself. If latter was the case, you can't call that code from another module without resorting to os shell level features.
The code encapsulated into "__main__" assumed to be module-specific, this is the purpose of that pattern. If you want to make some code available for another module, you just refactor it into separate function.
But the downside is that I like to use ipython and %run the file and if it's not in a function, I can then inspect all the intermediate variables and not just the result. Every time I do put it in a function, I regret it for this reason.