There are a lot of good reasons for putting interfaces in a separate package from implementation. For example you may want to use container that supports Dependency Injection to wire up the necessary implementation(s) at run time, in which case only the interface needs to be present at build time and the implementation can be supplied at run time. Alternatively, you may want to provide multiple implementations (for example using mock implementations for testing) or multiple versions of a particular implementation at run time (for example for A/B testing, etc). For these kinds of use cases it's more convenient to package interface and implementation separately.
