The muhapatti is a square of white cloth worn across the face and tied behind the ears or held. The purpose is to prevent saliva from coming into contact with sacred books, images or other items. It is often purported to be worn to reduce the chance of inhaling (and thus killing) small insects and other airborne life. Some Jains

Jainism

Jainism is an Indian religion that prescribes a path of non-violence towards all living beings. Its philosophy and practice emphasize the necessity of self-effort to move the soul towards divine consciousness and liberation. Any soul that has conquered its own inner enemies and achieved the state...

 believe the destruction of even these tiny life forms is a violation of ahimsa

Ahimsa

Ahimsa is a term meaning to do no harm . The word is derived from the Sanskrit root hims – to strike; himsa is injury or harm, a-himsa is the opposite of this, i.e. non harming or nonviolence. It is an important tenet of the Indian religions...

, the principle of non-violence. Whilst this may be one benefit of the use of muhapatti, it is not the initial reason for use.