An interface such as Map declares what an object can do. On the other hand, a class such as HashMap defines how an object does what the interface declares it does.
If you declare a variable (or field, or whatever) a Map, you are stating that your code depends only on the contract defined by that interface, it does not depend on the specifics of an implementation.
If you declare it a HashMap, it is to be understood that you need that specific version of a map (for whatever reason), and it cannot be replaced for something else.
I tend to dislike the common answers like "because you can change the implementation" simply because, after years and years of practice, you will see that it won't happen as frequently as that, and the major benefits are really this subtle (but clear) expression of your intents.
