Why do cosmonauts use hermetically sealed spacesuits when going into outer space?

Moreover, a certain air pressure is maintained in the spacesuit, because external pressure is zero. If the suit is not sealed, then all the air will evaporate.

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.