Why is the natural greenhouse effect important?
Without it the average global temperature would drop by 33 degrees.
The atmosphere is not heated by the sun, it is heated by the Earth. Incoming solar radiation primarily passes freely through the atmosphere and when it hits the Earth it warms the Earth. The warmed Earth emits radiation in the form of heat with is a much longer wavelength than the average wavelength from the sun. The longer wave length does not frees pass through greenhouse gases. This allows the atmosphere to not lose heat as quickly as it would every night.
The current standard atmosphere has an average temperature of 15 degrees Celsius. The estimated average atmosphere temperature without any greenhouse effect would be -18 degrees.