Larry Niven defines a macro-life civilisation as one that lives in interstellar spacecraft, independent of planetary resources and culture, full time and by implication keeps moving. This could mean small but self-sufficient generation ships or could entail something as vast as a ram-jetting Ringworld with a star for a fusion drive. Such a civilisation, once formed, would appear to have no resource reason to resettle on planets, (near asteroid belts yes; at the bottom of a planetary gravity well no), and in terms of long-term species survival macro-life seems, to me, the better bet.
The individual case answer will obviously vary based on cultural mores but I'm interested in the logical answer based on resources and long-term survival goals, to that end: Why would a successful macro-life civilisation give up their long held nomadic habits for a return to a planet-bound lifestyle?
Please note that for the purposes of this question it's all or nothing; either they colonise new planets or they stay interplanetary, travelling in habitats that don't need worlds and barely need star systems at all. Answers need to make a logical argument, based on long term survival, for planets being worth the effort of changing cultural norms that have existed for generations. The question makes the possibly unfounded assumption that there are suitable worlds at journey's end to be colonised at the option of these macro-life travelers.
Some contextual details that may have bearing on the question:
the macro-civilisation in question is composed of a small fleet, of ten to a dozen, very large (20+ kilometre long) slower-than-light generation ships, each of which has traveled roughly 25,000 light years when it reaches the colonisation target area.
the ships are capable of getting up to roughly 0.9C but accelerate at a maximum of only 0.2g.
any single ship is completely self sufficient of the technical equipment necessary to keep in resources and repair from material gathered from asteroidal and/or cometary debris.
any individual ship is capable of replicating itself over the course of a year of so given enough resources.
the trip has taken at least 4 very long generations for the ruling classes, several millennia at least.
the trip was initially undertaken for purely political reasons, the governors of the fleet left home rather than face the brewing revolt of their subjects and took large numbers of loyal subjects with them.
the governors have absolute power they can and will enforce whichever choice on the population as a whole, period.