Primary use is for soil and land (贫瘠的土地). Often extended metaphorically: 文化贫瘠 ('culturally barren'), 精神贫瘠 ('spiritually impoverished'). 贫 ('poor') stacks with 瘠 ('emaciated/lean'), giving a sense of land that is both poor and worn-out.
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贫 = poor, 瘠 contains 疒 (illness radical) + 脊 (spine) — 'sick land with bones showing'. Together: land too poor and weak to grow anything.
