Lots of music is performed first, and only later transcribed (often from a recording). In all cases, a music performance contains more data than sheet music (although there's no guarantee that the performer played their sheet music accurately to the page, if they had any). However, in many cases it's much more productive to learn a song by listening to a recording. Paul McCartney famously didn't use sheet music at all.

I guess sheet music is like documentation, but just like documentation it's often incomplete, wrong, confusing, misleading, or worse than nothing at all. Sometimes it's the only ground truth we have, such as with classical music where we can't know what the composer truly had in mind, so in cases like that I'd say the sheet music is effectively the source code.

Doesn't need re-explaining but makes me realise we often talk about comments explaining what code does, which leads to "your code should be well-written so it's easy to understand what it does" or "I'm a good developer, I can understand that easily by reading it".

But as you say the question is why - people will often be reading existing code because of a bug, so clearly the thought process that created it had a faulty step somewhere.