Greenhouse gasses are the thermostat (or an important part of it) of the planet. Dial up the greenhouse gasses and the planet’s temperature goes up. Dial them down and the planet’s temperature goes down.
The pre-Industrial Revolution thermostat setting was relatively stable for the last 15 million years and set the conditions that made it possible for many of the planet’s current species to exist. A change in that thermostat setting and the planet’s temperature will change what species can survive on the planet.
So greenhouse gasses are an important element of the current biosphere and a foundation for life on the planet.
Greenhouse gasses make the atmosphere function like a greenhouse by holding a certain amount of heat from the sun (much of the heat from the sun is reflected back into space). This ability to store heat makes the temperature of the planet’s surface conducive to many forms of life.
Since the Industrial Revolution homo sapiens have increasingly relied on the burning of carbon (wood, coal, oil, gas) and since the beginning have dumped the waste product (CO2) of this activity into the atmosphere. This dumping of CO2 waste into the atmosphere continues today and at higher levels each year. Currently, the levels of CO2 in the atmosphere are higher than there were for the last 800,000 to 15 million years. As a result the planet is now in the early phases of another mass extinction. This extinction is being called the Holocene extinction (current rates of extinction are estimated to be 10,000 times higher than a typical species extinction rate) and resembles, in terms of the changes to the atmosphere, the Permian-Triassic extinction event.