As a race, it would be our undoing to stay on the Earth forever. The Earth is an adequate place for now, but it is too susceptible to a natural disaster that could wipe out all life as we know it. At any point of time a meteor that eluded NASA surveying could pop out of the sky and strike the Earth throwing the world into a mass holocaust. If this did not destroy civilization entirely, then we would essentially be thrown back to the dark ages.
If its not a meteor, then it could be a black hole. Black holes are hard to spot because the only way to detect them is by noticing light being warped around them. A black hole, like a meteor, could invade our solar system and destroy everything with no chance of survival.
In addition to alien objects, a disaster could be cased by something on the planet like an earthquake, a tsunami, a volcano eruption, or a person. Life is fragile and it could end at any time. This is why we need to spread life throughout the universe in order to preserve it. If a colony could be established in several different galaxies, then when one suffers a race destroying disaster, then the life on the other planets will still exist. Those lost in the disaster will be mourned, but life will continue. The other colonies can even send aid and prevent the disaster from lingering on.
Having another planet that we could rely on for help is just one of the many benefits of colonizing the universe and just one reason why we need to leave the Earth. Even though we do not need to leave Earth altogether, we should put as many resources as possible into the establishment of a permanent colony in order to prevent against an all out extinction of the human race.
