We use the Past Perfect to indicate that a completed action happened earlier than something else:
Now look at the below sentence:
Why is the Past Perfect Continuous used here?
We cannot use the Past Perfect in the above sentence because the action was not completed. (In other words, she did not stop browsing the Internet immediately after looking out of the window.)
Why can't we use the Past Continuous? Because the action of browsing had started 15 minutes earlier than Jane looked out of the window.
So, if we were to remove "for fifteen minutes", we would use the Past Continuous:
To sum it up in one sentence: the Past Perfect emphasises completion (or the result of a completed action) while the Past Perfect Continuous emphasises the duration or activity of an action.
One final example: