Supposedly it is advertisement free (I wouldn't be surprised to see Warcraft related ads though) and supported only on subscription fees.
Also, for those who don't think it will add any value and they can read all they want online at different websites, just think a little bit how Blizzard can control the flow of information about upcoming changes to the game via their own magazine. Instead of the info showing up on their website first, Blizzard can publish previews for new content, class changes, etc. via the magazie (followed a week later on their website). They can also provide exclusive interviews with the game designers (themselves!). Couple that with some beautiful artwork and an in-game pet (you know they will probably do this like they do for everything else), there will be plenty of reasons people will be interested in subscribing.