The dangers threatening the Earth and humanity should not be our only motivation for the colonization of space. There are a lot of advantages of having space colonies.
Advantage # 1 : More territory and resources
In space there is more space and resources than we will ever need. There is a lot of territory and resources just waiting there for us to take. Even a single small-sized asteroid contains enough resources for the needs of billions of people. Space colonization thus might finally resolve most conflicts and wars in our world.
Advantage # 2 : Preservation of the Earth
When space colonization would be started, we would be able to move a huge percent of the human population in space, and thus save the Earth from overpopulation. This would in turn reduce the effect of global warming and excessive pollution.
Advantage # 3 : Spread of life in space
While colonizing space, we will surely bring with us all kinds of life forms. This would considerably reduce the risks of extinction of all the species as they will be present not only on the Earth, but also in a lot of separate space colony biospheres. Space colonization will also contribute to the biodiversity, since the life forms will adapt to their new environments. For example, when we will terraform Mars, we will be creating a second planetary biosphere with somewhat different life forms adapted to new planetary conditions.
Mars ice home. Mars has been the subject of terraforming ideas.
Advantage # 4 : More knowledge on astronomy
By living in space, we might gather much more knowledge about our new environments than with even the most powerful telescope from here on Earth. Also, telescopes in space are more efficient since there is no atmosphere to blur the view. The Hubble telescope is a good start, but it is rather small. Bigger telescopes could be built in Earth’s orbit or on the Moon. Also, the dark side of the Moon is the ideal place for radiotelescopes as all the radio waves from Earth are blocked by the mass of the Moon. Nevertheless, by reaching the distant planets and eventually the stars, we will learn much more than by merely observing them with our telescopes, would they be in orbit or on the Moon.
Astronauts installing telescopes on the moon.