There are quite a few elements that your lawn needs to grow. Some of these are found in the air and water, but others are in the soil and some need to be furnished to your lawn with fertilizer. All of your grass is fighting for the same food supply and when there is not enough to go around some will die or suffer - only the most resilient will survive. This allows weeds to move right in and take over the weak. If you want all of your grass to live and keep the weeds out, you need to fertilize your lawn.
Macronutrients or the big 3: nitrogen, phosphorus and potassium are the most necessary elements because your lawn uses them in great quantities.
Nitrogen is by far the most beneficial element for your lawn. It provides that rich, healthy, green appearance and aids in accelerated shoot growth. Nitrogen also escapes through the soil fairly quickly, so it is necessary to keep a good schedule for feeding your lawn. Beware of applying too much fertilizer because excess nitrogen can cause the lawn to "burn" and injure the grass.
Phosphorus is necessary for shoot and root growth. It is usually already available in the soil, but if a soil test reflects a deficiency, then you should fertilize. Phosphorus comes in the form of phosphate in regular fertilizers, sometimes taking a while to reach the roots due to slow absorbtion into the soil. It is best to solve phosphorus deficiencies before planting a lawn.
Potassium helps keep your grass healthy overall. It helps prevent disease and keep weeds out of your lawn. Potassium also assists your lawn in withstanding heat, cold, drought, and wear. Applying potassium on a regular basis can provide some insurance for your lawn.