OpenGL is a powerful graphics API available across the mobile world. You may associate it with 3D graphics and gaming, but its use if far broader than that. Simply using Z=0 makes makes it a powerful 2D graphics API.
Android uses it extensively, " all drawing operations that are performed on a View's canvas use the GPU.".
Similarly, your web browser uses hardware accelerated graphics.
Photoshop makes extensive use of OpenGL for image processing.
The bottom line is that OpenGL is more widely used than you might expect, and you can do some really cool stuff with it!
OpenGL is worth knowing, even without the 3D aspects. A quick look at the android documentation turns up the phrase "all drawing operations that are performed on a View's canvas use the GPU." Web browsers use the GPU to accelerate some CSS transformations and to composite different components into the displayed page. The GPU can even be used to accelerate numerical calculations. OpenGL is the API used to access and run code on the GPU. It is used indirectly through other components like the canvas, or directly where you need additional functionality, such as 3D graphics, or higher performance.
The GPU is even being used to accelerate databases!