It's not obvious in the example picture you posted, but sometimes doing the old style of mining, it was possible to end up with a material in the hopper that wasn't in any of the refineries, and in order to continue processing, you'd have to clear a refinery to make room for the new material, or eject the stuff in the hopper.
as an example, if you only had 4 refinery slots, and they contained indite, cobalt, painite, and copper, but then one of your collector limpets found a chunk of gold, you'd have to choose one of the four materials in the refineries, and eject them to make room for a possible gold process.
as an example, if you only had 4 refinery slots, and they contained indite, cobalt, painite, and copper, but then one of your collector limpets found a chunk of gold, you'd have to choose one of the four materials in the refineries, and eject them to make room for a possible gold process.