It would enable someone to query their knowledge graph.

Imagine I have a personal collection of recipes I like to cook, and the recipes link to foods with relation ingredient. When my friend comes over for dinner, I could ask them what foods in my kitchen they like, and instantly query all recipes containing those foods. Maybe I’ve added how long each recipe takes to cook, which appears as one of the many RDF triples I can use to filter and sort the matching recipes.

Knowledge graphs enable incredibly rich, structured queries. This is the promise and power of semantic web. Otherwise all you can do is rudimentary keyword searching.

The vision of the semantic web is to enable these rich queries over the entire collection of human knowledge, not just personal notes.