Thanks to the overly excited Greg and Daemon from IGN, we finally get some sort of clue as to what the touchpad will be used for. Sure, it's only for Assassin's Creed: Black Flag, but this could easily hint at what's to come with other titles as well.
The PS4 controller's touchpad will be utilized to navigate the in-game map. This intuitive process will allow you to bring up the map with a simple tap, swipe to navigate, and then pinch to zoom in and out, as you would naturally on a touchscreen phone or tablet.
While it certainly sounds trivial, at the same time, it will emulate how we all access maps on phones or tablets on a daily basis, making a task that would otherwise require the use of analog sticks, feel much more natural.
This is only one example, but if this is any indication, we should probably expect this similar functionality be implemented to other games that have an overworld map to navigate.