Some of it might be for ground crew
I've seen these on the underside of the fuselage in front of the nose gear. For example, UA
had "SP" printed on their 747SPs for that very reason.
I believe the original question concerned the ones which are visible to PAX, however. I don't believe it's a manufacturer requirement, but the fact that a specific font and format is typically used seems to indicate the "house" tends to promote the practice.
There is definitely some sort of "wow" factor involved, especially when descriptive names such as "Friendship", "Whisperliner", "Astrojet" are tacked on. However, it's been pointed out that AA
dropped the "DC-10" and left "Luxuryliner" after the trijet had its highly publicized rash of accidents.
Remember this not-so-subtle bit of advertising?
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Photo © Rolf Wallner