Is the knowledge of body language more useful for men or for women? Why?

Its very useful because this is a part of nonverbal communication and nonverbal communication is more than 70% part of any kind of communication.

Knowledge of body language is comparatively useful to both men and women as to men his body physique and guesture is what enhances his confidence to the people whereas for women the body physique and guestures attracts the people.

very important for both genders as it assists in many situations for example knowing how to break the ice when your meeting someone for the first time... etc 

Sometimes it helps you to take a good decision