What's the difference in pronunciation between "yo" and "you" in pinyin? I have no idea, for example, of what the difference in pronunciation between 唷 when pronounced with the first tone and 优.
If there's no difference, why are they written differently in pinyin?
There is a difference.


唷 (Toneless IPA: /jɔ/) rhymes with 波. The rhyme is the open-mid back rounded vowel, and is the ough in American English thought.
優 (/joʊ̯/) rhymes with 秋. The rhyme is the diphthong oʊ̯, and almost the same as ow in American English low.