Artillery Platforms were stationary battlefield defensive structures that could be captured by any commander from any faction. Artillery Platforms, apparently abandoned by their previous owners, could normally be found in tactically key positions, defending former military bases or oil production facilities from attack. By ordering their infantry (with the Capture Building upgrade) to capture these, generals could obtain a free defensive turret that fired shells over a considerable distance (normally far enough to hit artillery units). As usual, Black Lotus could capture these platforms for free. Artillery Platforms dealt reasonable damage to ground vehicles and infantry of all types, but unfortunately possessed a minimum as well as a maximum range. If a unit got close enough to an Artillery Platform they could capture or destroy it with impunity. Artillery Platforms were also unable to fire at aircraft.
Generally, the Artillery Platform was considered a useful tool to bolster defence if it could be easily acquired by a commander, but was otherwise not deemed essential in the same way as, for example, an Oil Derrick.
Apparently it was originally intended as a building for America. For one reason or another, it was decided to use the Fire Base instead. Note also that it used to consume quite a lot of power, while the Fire Base does not need any power which makes it useful in many situations.
The selection picture of the Artillery Platform is not found together with the other Tech Buildings, but among the American faction's selection pictures, which reinforces this.