Chemical fertilizers provide an “empty” type of food directly to the lawn. This is like the empty calories we get from eating pure refined sugar. Synthetic fertilizers do not contain organic material or bio-stimulants which feed soil microbes building the soil food web providing full service to the plants. They decompose dead plant and animal residues to humus; combine nitrogen and carbon to prevent nutrient loss; suppress disease; produce plant growth regulators; develop soil structure, tilth, and water penetration/retention; clean up chemical residues; shift soil pH toward neutral; retrieve nutrients from distant parts of the soil; decompose thatch; and control nitrogen supply to the plants according to need. Besides that, if a chemical fertilizer contains NPK of 10-10-10, nobody knows what the 70% of unlisted stuff is in the chemical bag that is not fertilizer. These fillers are not regulated and can contain some nasty stuff.