It is interesting to compare this with a similar sentence that only uses the Simple Past: 

In this sentence, both verbs are formed using the Past Simple. This puts them both into the same (past time) timeframe.  This grammar is simpler to form and therefore easier to use. It is the more common option, used when there is no need to emphasise an earlier time period or no requirement to eliminate any ambiguity about which action happened first.

Once again, like with Past Continuous, you can see that Past Perfect works alongside Past Simple. In this case, it is because there are two different time periods defined, both of which are in the past. The main action of the narrative is in the time-frame defined by the past simple verb. The action defined by the Past Perfect verb is in the earlier past, and it is giving background information that occurred before the events of the main narrative. As you can see in the diagram, the Reference Point (R) is on the completed earlier action. All information given about the time period before this reference point is narrated using Past Perfect.

Remember, Past Perfect focuses on actions which were completed before other Past tense actions happened.  For example, I had wanted to see that movie, even before you told me about it. It is important to note that there are two different PAST TENSE time periods in this sentence. Even if you are talking right now to the person who 'told you about it', they told you about it in the Past. The speaker's interest in the movie predates that discussion (Earlier Past).

Check out our sections on Combining Actions for more.