Associated types are a grouping mechanism, so they should be used when it makes sense to group types together.
The Graph trait introduced in the documentation is an example of this. You want a Graph to be generic, but once you have a specific kind of Graph, you don't want the Node or Edge types to vary anymore. A particular Graph isn't going to want to vary those types within a single implementation, and in fact, wants them to always be the same. They're grouped together, or one might even say associated.