Why do iron ore mines first stack the ore with stackers and then reclaim it via reclaimers to transfer it to export conveyors?

Stacking and reclaiming allow mines to create stockpiles of ore between stages of the operation. These stockpiles act as buffers that insulate the various stages from uneven flows upstream and downstream.

For instance, if the extraction of ore is temporarily interrupted, having a stockpile allows exportation to continue until extraction comes back online. Likewise, if exportation is interrupted, extraction can continue by just building up the stockpile.

In short, designing stockpiles into the flow, smooths out the flow by creating reserves along the way that can be expanded or depleted as necessary.