First and foremost, security
This individual escaped solely by chance. If he had not been wearing these safety glasses, the outcome would have been extremely different. Angle grinders are among the most challenging instruments to operate.
They are extremely noisy, difficult to regulate, and prone to a variety of accidents. While the grinding disc is not intended to shatter, accidents sometimes occur. This is precisely what occurred here. The disc may have split while cutting a piece of metal, sending flying pieces in all directions.