How is it that a lot of people with decent teeth get braces as kids?

To straighten them: Beauty is in the eye of the beholder. Parents often wish to give their children things they could not have as youngsters. What you consider decent, others consider unacceptable.

Longevity: Improved function and esthetics means teeth/jaws work better, last longer, require less maintenance, help gum/bone health, maintain jaw joint integrity. Orthodontics is viewed as a preventive service. Put teeth where they belong (best done while child is growing), and you will spend less money over your lifetime caring for them.