Laughter makes one exercise the muscles in the face (jaw and others). This exercise is very healthy,relaxes muscles and consumes energy.
Laughter makes ones mind free of tensions, pressures and worries temporarily. The mental health of the person increases. Laughter increases blood flow, which is good for heart and body, and balances emotions. Laughter also gives us some good memories.
Socially, laughter in a group of persons means we accept the interactions inside the group of members. Others feel your importance and presence. If you do not laugh while others laugh, it is treated as being impolite or not being friendly etc.