They're a traditional means of deterring cabbage white butterflies from dumping their eggs - and hence caterpillars - on your cabbages. Scatter broken eggshells of about the right size around the cabbages, The butterflies see all the white bits and interpret them as other butterflies laying eggs (or dead bodies that laid days ago, and hence whose babies have got a head-start), and hence too much competition for their eggs, and they move on to lay elsewhere.
Re-reading, that's a lot of hences, but you get the picture.