This article in Dutch, by a guy who stayed in Japan for one year working on Zen, mentions this “unnecessary thoughts”, mentioned by the roshi. For example worrying thoughts about the future, which may or may not happen, is basically pointless. He gives a short story about a samurai who got death sentenced and is staying in a prison cell. Next morning he will be killed. He cannot sleep and is walking around in the prison cell for hours. Suddenly he realizes that his death sentence is “only” tomorrow. What a nonsense to worry about it now. Especially when there is nothing to do against. He lays down and falls asleep sound right away. An extreme example, but it shows how this works. Our head can be full of thoughts which are not contributing anything really. The article gives also real life examples of the author how life is much more beautiful and interesting when our “head” is not in between. I really like reading these articles in this one year in Japan series.
Translate in your favorite translation software : https://boeddhistischdagblad.nl/boeddhisme/151312-sogenji-desu-en-dit-is-nederland-oranda-desu/