OpenGL is quite appropriate for 2D games. Although it is generally used for 3D, the same functionality can be used for 2D games. That is to say, anything you can do with 3D OpenGL will be applicable with "2D" OpenGL.
Some further information can be found at this location.
2D OpenGL is achieved in the same manner that 3D OpenGL is. 2D OpenGL is only the application of a technique, that is, rendering the scene on a flat plane, then using and orthographic projection instead of a perspective projection, which, depending on your setup, could distort the scene.
(a) is a scene in an orthographic projection. (b) is the same scene, using a perspective projection. In a 2D game, this can cause mis-positioning of sprites if their depth is altered (which, depending on how you go about development, may be the case). Using a perspective projection also makes aligning things in screen coordinates more difficult.
In addition, OpenGL carries many, many, many benefits with it. Primarily, hardware acceleration is a huge plus. There's also a much finer degree of control over what the graphics card does and how it goes about it, allowing for case-specific optimizations