The point is that you have a gear unsafe indication. It might look like its down from your vantage point, but the lights don't agree that it is safe to land on. Whatever the problem is, you might not be able to see it by peering through a window (especially at night), but it might actually appear more obvious to somebody on the ground with a broader view of your aircraft. It may not... but its common sense to get somebody to look at it in a low level flyby. I have seen this done dozens of times and the aircraft type is irrelevant. Before you commit to an expensive belly landing its worth considering all the options. The report stated that the pilot chose to land wheels up after the observers on the ground commented that the suspect leg did not look fully extended, so their observation was clearly relevant to the decision tree process.