Wasabi has a pungent taste which removes fishy smell from the ingredient, thus the fish becomes tastier many times as much. The most flavorful tip part and the spiciest root part of the radish are carefully considered to be used in order to bring out the best flavor. By grating wasabi with shagreen (shark skin), it gets more creamy and creates rich flavor. The sushi chef takes utmost care even for condiment.
Trivia 1 : Wasabi first started to be used in sushi around 1804 to 1818, and a popular shop in Fukagawa called Matsuga Zushi. It was used to eliminate the raw fishy odor of mackerel and its first use is actually documented.
Trivia 2 : At one sushi restaurant, Oyakata (the sushi master) grates wasabi clockwise, but Okami (the landlady) grates it counterclockwise. Learn more