"What you stay focused on will grow" - Roy T. Bennett
Law of Attraction is the concept that you attract what you hold in your mind. However, that is only part of the story. It is like paining a painting without any passion or feeling, no one will buy it, it lacks that something extra. The same goes for visualisation, if you don't add the feeling, there is no value. Law of Attraction is considered one of the Universal Laws, similar to the Laws of Physics, but for metaphysics.
The idea is that if you can see it in your mind you can manifest it. The problem is that the universe does not run a imagery facsimile service, it responds to the feeling that imagination gives you.
The visualisation is important to give an idea of what gives you that feeling. When you visualise what you want, you Reticular Activation System (RAS) kicks in. It is the part of the brain that lets the rest of you know that something is important and you then start noticing that everywhere. If you have visualised yourself on the beach for your holidays, you will start noticing adverts, news reports, etc about beach holidays, your RAS has told you it is important and thus you need to notice it.
The Universe serves up opportunities to feel more of what you feel in the present moment, so the visualisation allows you to evoke the feeling you want to feel in the future, in the present moment. So, essentially, you bring the future in to your present moment because you are already feeling the way you want to feel in the future. The trick here is to stay open to the opportunities and not be attached to the visualised outcome, remember, it is the feeling we are after, not the material manifestation. If we remain open to the opportunities, we remain open to receiving something even better that we couldn't even have thought of, but the Universe did in its infinite wisdom.
We do have to take into account our multidimensionality; who is it that holds what desire. There are many different aspects to you and parts of yourself can have conflicting desires and hold limiting beliefs about your ability to receive. If you are incongruent in this way your ability to manifest experiences you desire in your life is going to be limited. Being self-aware and working on your baggage will give you a far better chance to successfully manifest your dreams. Do the work and reap the rewards.