I have finally figured out why egg shells are added to soils, key word "finally". It has been a long time since I was confounded and baffled by this problem and I think I have the answer now until a long while of pondering. To understand how the eggshells work, we have to first look at the chemistry behind egg shells. If I remember correctly, eggshells are composed of calcium carbonate which can react with protons to form water, salt and carbon dioxide. As such, by adding eggshells, the carbonate of the shells can neutralise the acid in soils, thereby neutralising the acidity of soils. You may be wondering if there's any side effect in doing so. My answer to that question is no. Correct me if I am wrong but I believe egg shells are basically innocuous to the soil chemistry as it will remain unreactive in the soil unless the soil becomes acidic. Instead of being detrimental, the egg shells would actually be a boon in this case as too acidic a soil can hinder plant growth. With the acidity controlled by the harmless eggshells, the conditions will be right for the plants to thrive. If you think my reasoning is flawed, please feel free to comment and correct me. That's all I have to say in this post! Arrivederci and peace!