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Urabask
In my household, we hoard our eggshells for gardening. Eggshells are very useful for many purposes, especially in the garden: as fertilizer, pest control (excluding rodents), organic seed starter, calcium enriched soil/compost, lower acidity of soil, mulch...etc. You'd be throwing away a good mineral source of calcium carbonate if it goes in the trash. If they're baked and pulverized into a fine power, they're edible as a calcium supplement as well (although an excess does increase risk of kidney stones). Considering they're in another world, I doubt eggs are mass-produced, so every little bit of a food source is useful for something, unlike our current society that discards many things without a second thought. Adding it back into chicken feed can also help them replenish calcium for stronger eggs laid. Outside of gardening, calcium carbonate is a natural abrasive that can be used for cleaning.
Then again, she may just have thrown them away later off-panel ¯\_(ツ)_/¯