One use of the past perfect, according to Cambridge dictionaries, is where you're talking about a changed state, something that was the case but is no longer the case. For instance where someone promised something, then broke that promise. This can happen even when you're using the present tense.
They give a couple of examples:
A: Are you going anywhere today?
B: I had planned to go to the beach but look at the rain!
They note that here "had is stressed; the meaning is ‘I have now changed my mind’". It emphasises that B no longer wants to go to the beach, unlike "I planned to go to the beach but look at the rain!" In the latter case they might still go to the beach even though it's raining.
I’m very happy working as an engineer but I had wanted to be an actor when I was younger.
In the latter case the speaker is emphasising that they no longer want to be an actor, while "I’m very happy working as an engineer but I wanted to be an actor when I was younger." might suggest that they still want to be an actor even if they aren't one now.
It wouldn't be wrong to use the present perfect "...something they have promised to do", but "had promised" emphasised something that was done but is no longer the case: the promise is broken.