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Once a civilisation reaches the point where it can put reasonable quantities of manpower into space, it is generally assumed that it will start colonising other celestial bodies. However, as I understand it all the resources it needs will likely be available in moons, asteroids, comets, planetary rings, etc, all of which are way more easily accessible than those in the deep gravity well that is a planet.
So, why would a civilisation choose to colonise a planet instead of using those celestial bodies mentioned? Specifically, a colony that could be reasonably described as a town or city, not just remote research stations like we have at the poles.
Edit: This question is about why a civilisation would colonise any planet (regardless of whether they're in the home system or not) when moons, asteroids and other celestial bodies are much easier to access.