Bigger, better fruit that is the pick of the season.
Apple trees use a great deal of their resources producing blossoms, and, subsequently, apples.
If all the blossoms are pollinated, the resulting harvesting will include bushels of small apples that most likely will be discarded. Additionally, the trees' resources will have been depleted creating those apples. As a result, the blossoms that have been pollinated will set larger, more marketable fruit. If apple blossoms are not thinned, the tree will deplete its resources setting the fruit, which weakens the tree and affects the current and future year's production. This is called an alternate bearing cycle - alternate years of good and poor harvests.