Most information I’ve seen says that the ‘earthy’ smell is mainly due to geosmin, which is a compound produced by various actinomycetes and fungi. It can also affect the smell and taste of foods. In theory, the more you can detect the smell, the more active the soil biology is, which is a general indicator of healthy soil. Soil with no odor is generally very dry, which is limiting biological activity, or just has little activity to begin with and so is considered less healthy. On the flip side, any metallic/rotten egg/ammonia type smells is an indicator of anaerobic conditions, which is generally bad news for soil and would be considered very unhealthy.