Many people have written much on the topic of terraforming (changing a world to be more suitable for human life, more like Terra.) While early discussions focused on Venus or Mars, planets around other suns have been considered, as well as the large moons of Jupiter and Saturn.
Terraforming a planet is likely to require centuries or millennia at best, making it an unlikely venture. But in principle, the cost might be low (largely seeding with a mix of bacteria, algae, and eventually the rest of a viable ecosystem). It might happen, at least if the ethics of modifying another place to our liking is ignored.
What do you get if you successfully terraform a planet? You get a whole new world to explore and exploit, a new home for millions or billions of people. The downside is that it's at the bottom of a gravity well, which presently is a very expensive place to go to. Or rather, to get back from. It's easy to go to the bottom of a gravity well, of course.
Personally, I think it is easier, cheaper, and ultimately much more profitable to "terraform" asteroids and comets by hollowing them out and spinning them for gravity. You end up with much more room for people, plus a lot more resources for development. And these worlds are easy to visit.
Note that we are already busily terraforming our own planet Earth, or un-terraforming it, depending upon who you listen to.
Most would agree that we are in the middle of a process that can potentially change the surface and ecology of the entire planet. It is an experiment, and not a scientifically sound one (there is no control). It is driven by short term economics (of which population growth is an aspect), and fueled by the massive burning of coal, oil, natural gas, and (even more unfortunately) forests. Additional questionable experiments include the destruction of fisheries (and the unintentional but ongoing and significant evolution of fish stocks to make themselves less desirable to humans as a way to survive).
I will argue that this terraforming practice, while in most ways unfortunate and misguided, is still a valuable learning experience. Because someday, we will need to terraform our home planet in earnest. A new ice age is nothing to laugh about, it is serious business. Some day, we will have to warm up the planet to survive.