Might sound like a silly question, but I don't know why I should bother developing colonies beyond just being mining outposts or listening stations.
As far as I can tell, the amount of installations you can build on your home planet are infinite, and its exceedingly more convenient to make sure all your minerals are funneled into one place. At most, you'd want a single colony per system to act as a depot for your mass drivers to shoot their minerals at so its more convenient for your convoys to pick them up and ship them back to your home planet.
I understand that their is an upper-limit of 12 billion population on ideal worlds, but that number is extremely high, and with the default start of 500 million unlikely to be reached by most players.
I wonder if there could be an overcrowding or environmental degradation mechanic that could encourage the player to spread out their population and industries. I'd certainly like playing with an Earth that has far too many people who are dragging down productivity due to a lack of jobs and dependence on welfare all while the environment is making it harder and harder to sustain such a population. Would really incentive the player to get their ass to Mars ASAP as an early-game goal.
PS: I know there is a setting that heats or freezes Earth over time, but that isn't what I mean. I want super-concentrated industries to have an adverse effect on habitability that can be offset by tech, terraforming, or moving industries off-world.