As Joseph says in Session 7, “The Pursuit of Happiness”:
"It's important to include desire in the field of mindfulness, because when it arises unnoticed, we often are simply acting out that pattern of conditioning. We want to be aware enough to see when it's appropriate to act on it, and when it's more skillful to see the desire and simply let it pass away. Mindfulness gives us the space of choice. It offers the space of wise discernment, and it creates a space for greater happiness in our lives."
At this stage in the meditation, we're training ourselves to become mindful of the energy of wanting in the mind as just wanting, rather than acting on the desire. With practice, we can start to see how the energy of wanting comes and goes, and we don't HAVE to get what we think we want in order to be happy. In fact, specifically with addictions, acting on the craving usually brings short-term relief but long-term misery.
As you no doubt know, addiction is a very intense form of wanting, with many different mental and physical components to it. Because of the intensity and complexity, it takes a lot of practice to be able to see all its different aspects clearly and train oneself to not act out on the craving. Fortunately these days there are books and programs and communities that offer support for using mindfulness as a tool in recovery from addiction. There are too many to list here, but if you just Google "mindfulness addiction recovery" for example, you will find many resources.