|The following is based on Generals cut content and has not been confirmed by canon sources.|
It was originally designed as a convenient-looking power plant for the low-on-budget GLA, but the developers have decided to give the GLA the advantage of having no need of power plants. The Windmill was supposed to be the GLA's only power source of the GLA. It was to create an average power supply.
However, there is still quite a lot of files related to the windmill left over in the game's code.
The windmill appears in the Zero Hour Rise of the Reds mod as the GPS Scrambler Station, as well as in the Generals Nova mod.