In the early history of tube artillery drawn by horses (and later by mechanized vehicles), ammunition was carried in separate unarmored wagons or vehicles. These soft-skinned vehicles were extremely vulnerable to enemy fire and to explosions caused by a weapons malfunction.
Therefore, as part of setting up an artillery battery, a designated place would be used to shelter the ready ammunition. In the case of batteries of towed artillery the temporary magazine would be placed, if possible, in a pit, or natural declivity, or surrounded by sandbags or earthworks. Circumstances might require the establishment of multiple field magazines so that one lucky hit or accident would not disable the entire battery.