There are mainly two reasons why Fresh Wasabi is used in Sushi.
First, Wasabi has a strong anti-bacterial effect. Eating raw ingredients including fish increase the risk of foodborne illness derived from bacteria. Wasabi is a potent anti-bacterial agent and fights against some of the bacteria from raw fish. “Research has shown that such isothiocyanates inhibit microbe growth, perhaps with implications for preserving food against spoilage and suppressing oral bacterial growth.”
Second reason for using Wasabi for Sushi is its aroma and flavor. Freshly ground Wasabi has a pleasant and refreshing aroma and mild to a hot burning sensation. Wasabi goes really well with Tuna (Yellowfin, Bigeye, and Bluefin) creating complex taste with fish and soy sauce. Some of the medical benefits of Wasabi include, Antibacterial effect, reduces the risk of cancer and cardiovascular disease, anti-inflammatory, improves the immune system, subdues Arthritis, improves bone strength, helps respiratory system, boosts digestion, promotes appetite, prevents cavities.
Lastly, wasabi is good for your brain. There is a study done by a professor at Nagoya University suggesting that eating Wasabi promotes the reproduction of brain cells, improving memory and learning the ability. Professor Okajima ran a four-week study on mice with the diet containing the same hot chemical compound found in Wasabi. The mice showed improvement in brain function compared to the ones without the hot chemical compound in their diet. If translated to human consumption, it is about 12.5g (.44oz.) of Wasabi a day.
Be minded that too much Wasabi consumption could cause diarrhea and numb your taste bud.