Spoiler-ish.
At the end of this dream (for example in secret H174), you are given an influence card to add to the deck of a player and another one to add to the storage.
I assume that it doesn't mean that it can be deckbuilding into a deck but only serve as a reminder. Is that right?
I assume this card is useless in storage (until specifically called to check for it in the future, which can makes sense as it might be a way to mark a choice made in the past).
Self answer: those cards cannot be used, they just stay there. They come up later, as expected.
It's a great use cards to have a choice being impactful later on.