I'd like to think of the entire hull to be self-illuminating. Not much, but enough to be seen in the stark blackness of deep space.
As for who sees it, well... first of all, most windows offer at least a little piece of the hull in the view. Psychologically, I would say it would keep the shipboard crew grounded, even when not in a star system; to remind them that they're a part of a bigger whole. Then there's the whole idea about Starfleet not sneaking around; they are soaring proudly in the depths of space. At the least, the visual scanners of other ships (even possible enemies) should be able to get a decent picture instead of merely a heat-related visual.
EXACTLY!, and I might add, even "in-universe" I feel peeps would still add aestethics-based design touches to ships for the sole purpose of aestethics-based design. There is a technical aspect and/or reasoning and/or design behind applied psychology as well. The parts effected just can't be held in one's hands. Nevertheless, they are just as real. You can't hold a thought in your hand, and yet: Does that render it not-real, and thus beyond consideration?
Do stripes and/or paint-schemes on aircraft serve any other purpose beyond the psychological? No. And yet, there they are.
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