This piece is called “Darkest Before Dawn,” and I'd describe it as space music, although others have described it as dark ambient. I listened to this piece while meditating just last week, and found it to be very helpful for that purpose.
The reason for using meditation music, in my mind, is to drown out incidental sounds that might otherwise be distracting and break your concentration. Really good meditation music can also have the effect of triggering your subconscious mind, as changes in the soundscape tend to produce sudden changes in your field of consciousness. Of course, the benefit of this depends on what type of meditation you engage in, as some methods of meditation are aimed at stopping thought rather than playing with it.
I personally aim to play with consciousness in order to learn new things and have new insights as a result. My meditation is also focused on dreamwork. Both of these types of meditative practice can be benefited by listening to this type of music. Steve Roach's music does more than just help you silence all the superfluous noises. It also helps send your mind off in new directions and down unexpected paths and streams.