Posted: 21/07/2015 at 14:59
The reason for dead-heading is to stop the plant doing what it's here to do - i.e. produce seed for the next generation, then die (or die back).
The reason for this is that if you remove the 'dead' flower head including the green bit at the base of the flower (the seed pod), the plant tries again to produce seed. So you get more flowers.
Generally speaking (there are exceptions) once the plant has flowered and the seed pod is ripening, from the plant point of view, it's job done and they can happily die knowing they've produced seed for the next generation. By keep removing the seed pods the plant has to produce more flowers to try and 'set-seed' again.