Over time, many lawns will leech nutrients out of their soil. Most nutrients for your lawn are water-soluble. Over time, whether through rain or irrigation, the water will carry these nutrients out of your lawn. This will leave your lawn nutrient deficient, which will leave you with an unhealthy lawn susceptible to disease.
In some instances, the soil that the grass is growing in does not have the naturally occurring nutrients that turf needs to grow. If this is the case, fertilizer will be needed in order to maintain a healthy lawn.