viviparous animals like birds have a disadvantage compare to viviparous animals like mammals (humans) because the mammal embryo will have a steady supply of nutrients throughout its development through the umbilical cord. The egg needs to have all the necessary nutrients to shape the organism.
The egg yolk along with the egg white supply the "raw materials" necessary for the embryo development.
Protein is used to make structural component like muscles and bones also enzymes are a type of protein.
Fats and cholesterol provide energy. Cell membranes are phospholipid bi-layers. Lipids are fats hence the need for them. Some hormones also contain protein and lipids.