What a great question.
As children age the jaw will continue to grow. It can therefore be really beneficial for an orthodontist to utilise this growth in your treatment. So, starting orthodontic treatment really young, sometimes the age of 5 can help children develop the space required to have straighter adults teeth… Without having to have orthodontics as an adult. This is known as preventive orthodontics.
Straight teeth have also the tendency to be easier to clean. When your child cleans between their teeth that are likely to miss difficult to reach areas. Crooked teeth have lots of difficult to reach areas and can therefore be much more difficult to clean.
And of course there is the cosmetic implication. With the modern influence of social media, children look to have straighter teeth in order to feel confident.
I do hope that helps