They manage the fundamental components of a scene/world to be rendered
- Points, lines, light sources, cameras, viewports, materials
They provide transformation functionality
- Scaling, rotation, translation
The provide window management functionality
- Creating, destroying, moving, resizing windows
The provide input management
- Keyboard, mouse, controller, etc.
They interface directly with the graphics drivers to send information to the graphics card

Any or all of these can be replaced by functionality from the game engine itself (though if it is all replaced then there is little point in using the API!).

At a minimum the API is used to interface to the graphics card hardware, freeing up the game engine developers from concerns about compatibility with different graphics hardware.

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