A lot of modern games are not very complicated, and don't require you to know or remember much stuff. The ultimate example of that are games with Quicktime events, where you don't even need to remember what button to push, the game will tell you. On the other end of the scale are simulations and open world games, where remembering where stuff was, or knowing the average price of a tradeable good is quite necessary for success. In the past I used a second screen on my PC where I could display such information. But today the more modern version of that concept are companion apps.

For example I recently picked up Assassin's Creed IV : Black Flag at a Steam sale. Nice game, except that is crashes to a black screen from time to time. Anyway, the game comes with a companion app which you can install on basically any tablet or smart phone. Once you synchronize the game with the app, you have a second screen for all sorts of information. Most useful is the tablet holding the map of the game, updating your position in real time; you can even select your next target on the tablet and get the marker beamed back into the game. You can also check the loads of information in your database about people, locations, documents, and so on which you found in the game, again updated in real time.

I know that other games have companion apps as well, for example Destiny or Titanfall. In the new World of Warcraft expansion, you can manage your garrison with a companion app. In that case the idea is that the WoD garrisons work a bit like mobile games, where you are supposed to do some small activity from time to time, and having access to that on a mobile platform is helpful. Watch Dogs has a companion app which lets you control the police and play against your friends who are on their PC/console. So there are a range of different concepts for companion apps, some more useful than others. But as there are less and less people out there without some mobile device, I can only presume that we will see a lot more companion apps for games in the future.