Imagine trying to me sense of Beethoven's String Quartets from the printed page - I don't think so

Beethoven managed it.

Blacking describes what Grainger's notation was trying to do in his book. It's less necessary now that recordings can be more permanent, but the effort to describe a performance from the outside as precisely as possible is something future generations will always be grateful for. The modern terminology for Grainger's notations is "etic" - the contrasting "emic" notation that performers might use themselves is more useful to them, but not a lot of help when we no longer have performers who can understand it in the same way as the person who wrote it down.