Why during the Great Patriotic War soldiers were advised to open their mouths in case of strong explosions?

In order to balance the pressure on the eardrum from the inside and outside, otherwise the blast wave may damage it or cause severe contusion.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.