Recent trends have shown that having the entire human race residing on the Earth is not a feasible strategy for the survival of said human race. We must therefore start to expand our living space in an ethical manner. Terraforming dead or long term uninhabitable planets is an ethical manner of expanding our living space.
The concern about the need to do this is uncertain, but we would do well to get started on it. You cannot just terraform and colonize a planet overnight. There are probably many roadblocks that would be encountered, including totally unexpected complications. It is something that it would be unwise to delay, and then be sorry later. We are just at the stage of considering how humans would tolerate the trip and considering sending two people on a trip to Mars. That is a long way from the ability to send a colony of people who could actually sustain an existence there. It's necessary that we continue as if it is a priority, even if it doesn't turn out to be one.
First of all, to colonize things like the Moon or Mars is way more expensive than any country can afford right now. Also, there is still plenty of space on the Earth to fit a growing population. If people are concerned about resources, other planets will not help with that because, obviously, those places do not have the resources to support human life to begin with.
The so-called "death of Earth" is greatly exaggerated by the media. Earth has gone through a variety of changes, and the ones we are experiencing now are no different. However, while I don't think there is a "need", I don't see why humanity wouldn't "want" to expand and explore the resources of other planets.
There is enough room for everyone in the population. The problem is that it is not well-distributed; in some places, people are packed into uncomfortably small areas, and in other places, there are acres and acres of unused, inhabitable land. As far as resources, studies have also shown there is more than enough food for everyone on the planet; again, many countries are without it, and others have an excess - for instance, in the US, where so much food is wasted.
I also believe a lot of doomsday theories about meteors or asteroids hitting the earth, nuclear winter, etc., are grossly over-hyped for the media to make money.