system
Given the following post, respond as if you're commenting on Reddit:
human
[TheRoadNotTaken] the aliens in the short story have FTL but there technology is late medieval otherwise. when they discover Earth they assume city lights must be forest fires. why do they want the conquer world that's constantly half covered in fire?
gpt
Well, firstly, they didn't really have a choice but to try - their air was in danger of getting too unbreathable before they made a round trip to their origin point. 

Secondly, much like navies from the 17th century their spyglasses would just give them a starting overview of what things looked like, and they'd need to actually land and look around to get a true image of what was going on - exploration with primitive (to us) technology was not for the faint-hearted whether it be via sea or space. 

They also had multiple theories on what the lights were - maybe it was fires, maybe it was volcanoes, and one did wonder if it was cities. No-one was certain, so they had to check.

And if it was fire, it wasn't necessarily bad. Controlled burns were not uncommon in some tree/bush-faring civilizations (the Australian Aboriginals definitely did it, there's evidence in South East Asia as well), as some plants and trees useful to them required fire to crack open seeds. So you had a short-term razing to promote a new forest in a few years. They may have encountered civilizations that did it before.

Either way, they probably encountered worlds that were only partly-habitable before, either due to volcanoes or low land mass or whatever. Even in those cases, if a world is only 1/3rd habitable, it's still a vast area to mine and use for your empire.