A part of this billboard sign is written horizontally (the 自助 part) probably due to space limitation (reducing the text size was not considered — wise move).

But the nice thing is, it works. The two idiograms 自助 represent self-help – 自 for self, 助for help – help yourself. Add 餐, which means meal, we have buffet – 自助餐. Unlike English, written Chinese are made up of words, not characters, which make Chinese written language extremely compact and efficient. Less space, no character tweaking. Let's dig more into this topic…

• Less space: I love her -> 我愛她. Even if we give two English character space for one Chinese idiogram, the Chinese version already takes four character spaces less in such a short sentence. Imagine what will happen in longer sentences…
• No character tweaking: The words themselves do not change. In English: I love her -> She loves me. In English, just switching the perspective already requires modifications on all three words: I to me, love to loves, her to she. In Chinese, 我愛她 -> 她愛我. Flip it, and you're done! Isn't it logical? Isn't it THE way things should be? People always lament about how difficult it is to master the four different accents of the Chinese language, but that is nothing compared with this convenience.

Plus, as proven in the billboard, we can mix horizontal and vertical text direction if we need to. Not that it is much useful, but flexibility and "having another option" are never bad things.