Japan is a country full of ancient traditions and eating in a Japanese restaurant is no different. It is not to say that you must follow every single one of the Japanese etiquettes to the tee but wouldn’t it be nice to know and to be practice some of them when the opportunity presents itself? Plus you could really impress someone of how worldly you are.
The position is called “seiza”, which is where you would sit in a kneeling position with your behind rests on the back of your ankles. It was originally practiced in the Samurai era to show respect and honor those that are sitting with you.
To be fair time has changed and since the seiza position can be quite uncomfortable modern Japanese people don’t really go out of their way to sit in such position anymore. But there is a time and a place for everything so practice your seiza and you can impress next time you find yourself in a traditional Japanese restaurant.