The atmosphere ensures that all living things can carry out daily processes that are vital to survival, such as breathing. Photosynthesis, for example, could not be possible without an atmosphere, because all of the gasses in our atmosphere stay there due to Earth's gravity.
The atmosphere is vital because it plays a role in the water cycle as well, allowing rain to keep falling and giving life-giving water to organisms that need it.
Life would not be possible without an atmosphere on this planet, along with other vital things, like gravity, sunlight, and water.