We only have a limited amount of space on Earth, and much of it is taken up by oceans or ice fields, is too far away from population centers to be useful, or is polluted. It makes sense to reclaim polluted lands whenever possible. The process is quite simple: improve land that has been disturbed by destructive activities until it is in a state to be used for the designated activity, or even until it is equally capable as it started before the destructive activity damaged it.