Seed and fruiting structures develop immediately after bloom. Such processes consume valuable resources that could otherwise sustain subsequent bloom or vegetative growth. That is why it is helpful to deadhead some of the many blooming plants that do not need to produce fruit or seed. Deadheading diverts resources to more constructive application.
Deadheading is simply the removal of deteriorating bloom prior to the maturation of seed or fruiting structures. Besides diverting resources, it removes unappealingly deteriorated bloom, as well as unwanted or potentially invasive seed. Deadheading can be delayed if seed from particular flowers is desirable, (although some types are genetically variable).
It was time to deadhead spring bulbs as they finished bloom earlier last spring. Now it is time to deadhead some of the summer bulbs. It eliminates unsightly faded floral stalks of gladiolus, and diverts resources into developing bulbs. It eradicates invasive montbretia seed. For canna, it conserves resources to enhance subsequent bloom through summer.