Example: You're looking for a house. There may be several databases of houses for sale in the area you're looking. You want one that's within walking distance

Example: You're looking for a house. There may be several databases of houses for sale in the area you're looking. You want one that's within walking distance of a good school; the regional government has a database of schools and data about them. If those databases are published in RDF (or some machine-readable format that can be transformed into RDF), then it's trivial to smoosh them together and query them as one.