One of the original goals of Unicode was to be able to computerize every document. I still have some old linguistics books in which characters have been handwritten into typed or even typeset text. So these are the types of documents being referred to: academic papers.
Some fancy books have photographs of ancient writing; I’m not sure if Unicode tries to encode such sources and I pretty much doubt it (how would you even know what to call the symbols? You touch on this in your comment). However often they are attached to treatises that order the characters in some way (I.e. index an alphabet) in which case the first case above would apply.
In other words: thanks to some scholars who wrote down and ordered runic alphabets, you can now discuss runes with your friends and colleagues through email.