Deadheading enhances continuous flowering. Once annuals start to bloom, they will not stop producing flowers until seeds set or a frost kills them. Some examples of continuous blooming annuals that benefit from deadheading include marigolds, impatiens, geraniums, and zinnias. Perennials tend to have a shorter flowering season, but they can benefit from deadheading to extend the flowering as long as possible. Encouraging re-bloom will make the growing season much more enjoyable.
Some examples are coreopsis, rudbeckia, and Shasta daisies.
Deadheading also has been known to encourage a later second bloom for some perennials like delphiniums. The importance of deadheading in relation to the overall health of the plant is readily understood in terms of energy conservation. By preventing the production of seed, the plant is allowed to put more resources into root and foliage growth, as well as the production of flowers. In the case of some annuals, seed formation can mean a rapid demise of the plant, so energy that would go into seed formation is used instead for more flower production. Perennial plants that are deadheaded before seed production will use the energy towards the development of next year's flowers.