There are certain types of malocclusion that are have been shown to adversely affect quality of oral health and quality of life, but these are far, far fewer than the number of people sporting braces, adult or adolescent. The need for braces in the average teen, therefore, is no greater than the average adult. If you think of braces for teens as a necessity, it is no less a necessity for adults, that is, appearance is probably the only significant result for both average teens and adults.
However, increased longevity has led to the need to keep healthy teeth for longer periods. Teeth continue to shift throughout life. Missing teeth (today's older adults are more likely to have had dental extractions than younger adults) allow teeth to shift, resulting in malocclusion. Malocclusion in turn can result in frequent inadvertent oral mucosal injuries. This can be corrected with braces.
Missing teeth also mean less occlusive surfaces (chewing surface area). Believe it or not, this poses a choking hazard, as well as decreases a person's ability to enjoy certain foods. Braces or other orthodontic devices may be needed to shift teeth into proper position for bridges to restore occlusive surfaces. Since people are living longer, it makes sense to invest in procedures that will help you to continue to eat well.
Recent reviews of dental literature, however, are showing that the oral health benefits of orthodontic intervention in adults are more limited. Malocclusion seems not to have a role in susceptibility to dental caries (as was previously believed), periodontal disease, and temporomandibular joint disorder. Therefor the oral health benefits of orthodontic intervention are more limited than previously believed.
Finally, there is some evidence that adult orthodontics can result in harm in the form of root shortening, which can lead to loose teeth.
Determinants of masticatory performance in dentate adults
Dental crowding as a caries risk factor: A systematic review
Causal relation between malocclusion and caries