Exactly. What happens if we have a swarm of satellites to divert/concentrate a large amount of solar radiation? What happens if we geo-engineer bacteria to precipitate out greenhouse gases? How long does it take for particulate matter to settle after diverting an asteroid to impact the planet? If we can make oceans, how does the size of oceans affect the absorption of new asteroid deliveries?

Mars and Venus are, as far as every experiment has shown, dead as doornails. Using them as a testbed for experiments that are too dangerous to do on Earth without first practicing is a good use for them.