SomeCache is a real, functioning implementation. Maybe cache is a bad example. It could also be a HTTP client to fetch some data over an api. Let's say the lib

SomeCache is a real, functioning implementation. Maybe cache is a bad example. It could also be a HTTP client to fetch some data over an api. Let's say the library by default uses Guzzle. But if a user for some reason wants to inject his own implementation of Guzzle or even BuzzBrowser (which is compatible for basic operations) instead, he can. For most simple projects, especially without a framework, the default implementation does the job and would save you some code.