Off the top of my head, here are some things associated with the platform layer (in HMH):
- Opening a window
- Getting an OpenGL context
- Allocating a chunk of memory to be used for the game
- Getting access to a sound buffer
- Processing input messages from the OS
- Allocating threads
- Accessing files
- Watching for updates to the DLL where the game code executes so that it can be reloaded as the game runs
Also, there's some stuff I haven't kept up with lately such as getting timing information, getting monitor refresh rate, etc. The platform layer does a lot of the same things something like SDL (Simple DirectMedia Layer) would do.
Hope this helps.