Atmosphere is necessary to sustain life on earth because without atmosphere there would be no oxygen, no weather changes, rainfall, etc, which are essential for activities of human life and ideal temperature on the earth.
The atmosphere protects the earth from harmful ultraviolet and infrared rays of the sun. It has oxygen and nitrogen, the life-sustaining gases. It helps in retaining the necessary warmth on the earth and helps in the circulation of water vapour as the source of rainfall. These functions of the atmosphere keep it in a dynamic state.