There are two major reasons why deadheading is an important garden chore. First is aesthetics. The plants simply look more attractive when the old faded blooms are removed. Spent blossoms can be unsightly, and removing them will not only clean up the plant, but could prolong the blooming period as well. Another reason to deadhead is to prevent the plant from producing seed.
Plants produce flowers for reproduction.
Seed formation is the ultimate goal of most flowering plants. Interruption of this process through deadheading can encourage more flowers to be produced as the plant will once again begin the process of reproduction. This is especially true for annuals that bloom continually all season long. Deadheading enhances continuous flowering. Once annuals start to bloom, they will not stop producing flowers until seeds set or a frost kills them.