Bunch of comments so far, so let me take a stab at why this chart says what it says...
Currently, there are about 40 million PS4 sold, and by lack of proper numbers from Microsoft, one can speculate that there are about 20 million Xbox One's in the wild. Speculation also points to over 125 million users on Steam, so it's safe to say that the PC gaming market is close to 150 million or a bit more.
Even if you put all of these platforms together for a total gaming market of approximately 210 million players, it is nothing even close to the Android and iPhone users, with 1.4 billion Android and 505 million iPhone users. This is a mobile market of almost 2 billion, or about a quarter of all the people on the planet. Just by sheer volume of the market it is easy to say that mobile gaming is much more "important" than any of the more traditional platform market.
But what does this "important" mean exactly. Well, as far as what I would guess, it is the strategic importance to the business of the developers. Mobile is more important than any other platforms for various reason from a business strategy position.
Firstly, as I've already shown, there are an insane amount more users of mobile platforms. This market is also growing at phenomenal rates, and more and more money will be generated from this incredible vast amounts of people
Secondly, micro-transactions makes for a highly lucrative income stream, as the involvement psychology of paying small amounts for in-game items is much easier for consumers to justify than spending a big amount upfront for a game. More people will justify paying a small $1 amount per item as spending $60 upfront. This is evident in the research most people do before buying a new game, where paying for micro-transactions have a very low involvement.
Thirdly, overheads and production costs are much, much lower in creating mobile games than traditional games. This means that margins will be much higher in mobile gaming than other platforms.
Fourthly, expected growth in the mobile gaming industry is excepted to grow year on year for the next foreseeable future. People all have phones, and even though it will never become the main gaming platform, it will be a secondary gaming platform for billions of people around the world
All these points can further be the reasons why the biggest gaming companies in the world at the moment are all focused on mobile gaming. It makes so much business sense that mobile gaming is more important than any other platform.
Also, remember, just because you may not play games on your cell phone does not mean its not a viable gaming platform. The fact of the matter is that even if only 1% of all cell phone users play games on their cell phones, the market would still be almost as big as the Xbox One platform, and lets be honest, we know that there is much more than 1% of cell phone users that play games on their phones. And the more games people play, the more likely it is that they will pay for micro-transactions. Also, remember that the youth of today will be the working, income earning adults of tomorrow. And children love playing games on cell phones. Now imagine if this generation grows up with their own income, the mobile market will explode.
Now is the perfect time for game developers to get into mobile gaming, and now if the perfect time for them to make mobile gaming more important than other platforms.