In many production systems you'll want to have one module capable of talking to many potential implementations of a collaborator module (e.g a in memory cache, a redis-based cache etc). While testing it's useful to control which module the module under test is talking to.
Here are the approaches I can see. The two points that seem to divide the approaches are their tool-ability (dialyzer) and their ability to handle stateful implementations (which need a pid).
Modules are first class, so you can pass them in. Used in EEx, where passed module must implement a behaviour.