It was a naturally-occurring phenonemon which helped us increase the temperature on earth, if temperature get too high, there will be much disaster occurring on the earth.
There are some man-made greenhouse gases such as Fluorocarbons or carbon dioxide, they had the ability to trap heat which warm the surface on earth.
TWO IMPORTANT greenhouse gases. ozone and halocarbon.
I believe that greenhouse gases are important to us, we need them to keep temperature constant. Exceeding limit of greenhouse gases will cause a lot of environmental problems. For example, Ice will melt in a faster rate causing flood and disaster for MAN.