If unfed, they'll grow sparsely, allowing weed seeds to germinate and flourish. Grass that's insufficiently nourished is also more susceptible to pests and diseases.
Spreaders make it easy to distribute fertilizer evenly over a lawn.
What your lawn doesn't need, however, is year-round, overenthusiastic feeding. In fact, some say the greatest threat to our water supply comes not from pesticides or industrial runoff, but from excess nitrogen applied to lawns by home gardeners and landscape maintenance companies. Extra nitrogen fosters algal overgrowth in lakes and rivers, thus upsetting their ecosystems. Beyond this, it doesn't even help the lawn that much in the long run. More thatch develops, roots don't grow as well, and more water is needed.