Chrome OS is currently a bit awkward for some use cases in tablet mode, but fullscreen apps and games are about to become a little bit easier to use thanks to the adoption of the Immersive Mode option that's been around in the Android world since Android 4.4 (KitKat). According to Kan Liu, Google's director for product marketing, code for Immersive Mode will be pulled from Android and adapted right into Chrome OS' codebase, allowing apps with support to go fullscreen in tablet mode, and give users an option to drag down the top bar to exit the app or go to another. This is in stark contrast to the current way that one has to go about using Android apps in fullscreen on a Chromebook in tablet mode, which is putting the app into fullscreen while in laptop mode, then flipping into tablet mode. Vice versa applies when exiting the app, of course.
Chrome OS' implementation of Immersive Mode will not be exactly like that found in Android. It may not only apply to Android apps, but that has not been confirmed or denied by Google at this point. What has been confirmed is that there will be no way to force immersive mode at launch. While this option may come in the future, for now, any apps that want immersive mode will have to be coded in by the developers. Most Android apps that need it, such as games and video apps, of course, have already had Immersive Mode support for quite some time.
As one could imagine, the current setup has made using tablet mode somewhat difficult, especially with Android apps. This change not only helps to make convertible Chromebooks like the Samsung Chromebook Pro and ASUS Chromebook Flip more appealing, but also opens the door for Chrome OS to find its way to tablets. Without a reliable, intuitive fullscreen option, much of the point of a tablet is lost. Tablets are, at their core, media consumption devices, after all. Chrome OS has been shown to have code in the works that could support a tablet form factor in the future, so working the kinks out early on, will likely prove crucial for greater adoption levels.