Why are greenhouse gases important to life on earth?

They harm the atmosphere which is bad for the earth since the atmosphere contains all the air

Most of the Earth's atmosphere is made up of nitrogen and oxygen, which do not have much effect in regulating the climate. Other gases that occur in trace amounts (<1% of the atmosphere) have a much bigger impact even though they occur in relatively small quantities. These are known as the greenhouse gases.
Energy (light) from the sun passes through the Earth's atmosphere and is not absorbed by the greenhouse gases (due to its short wavelength). The Earth absorbs this energy and radiates heat energy at a longer wavelength (infrared radiation) back into the atmosphere. The greenhouse gases absorb some of this energy and radiate much of it back towards the surface whilst the rest is radiated out to space. This plays an important role in keeping the Earth's surface warm and able to sustain life. Without this greenhouse effect the Earth would be much colder and life on this planet would be very different. This effect is called the greenhouse effect, because it acts a bit like a glass greenhouse that traps heat creating a warmer environment inside the greenhouse.

That is true! many fish species traveled to the u.s. because of it.