Organic soil is the same stuff that you wash off your dinner plates, grimy stuff that started out as a living thing. Proteins, starch, oils. All of that stuff is in wort/beer. Without using any kind of caustic cleaner (PBW), it will build up because some of that stuff is not easily soluble in plain, neutral pH water.
Inorganic soil is mineral based and needs to be dissolved with an acid to be rinsed away. Beerstone buildup happens to all brewing equipment both on the hot side and in kegs. If the inside of your kettle doesn't look shiny clean anymore, that's beerstone. When it's on stainless steel, you can remove it with elbow grease and bar keeper's friend or just nuke it out with hot water and citric acid.