Meditation is a key factor in the development of skills of many esoteric sects. It helps to calm the mind, and bring the spirit into harmony. Meditation can be used to divorce one's self from their sense-of-self, and bring them more in tune with the rest of the cosmos. Meditation can help to bring about such things as visions, and insight from the universe, and whatever Higher Power one follows.
But, these are not concrete answers as to "Why Meditate?", so I will start actually answering the question.
Meditation is, in its simplest form, sitting still and not doing anything. It can be done with the eyes closed, or with the eyes open. It is either very in-depth thinking, or not thinking at all. This is an exercise that is pivotal in the development of all people, whether they follow an esoteric calling or not. People have to sit down and shut up every once in a while. It recharges you. Without meditation, we would never get anywhere on anything. We would not be able to step back and look at things as they are.
Asking "Why Meditate?" is like asking "Why Breathe?". Meditation is needed for healthy living. We all need to sit back a moment away from everything once in a while. Without stillness, how would we ever hope to move? Meditation is one of the most effective ways to bring the mind, body, and spirit into harmony. When that happens, our lives become life, not work.
There are so many things that can be said about why one should meditate, it is difficult to know where to start. Immediate results will include such things as increased energy, decreases stress, emotional balance, mental clarity, and other things of the like. More long-term affects include things like an altered state-of-mind, continually sharp mind, true emotional balance, unique understanding of life, and enlightenment.