The main reason is to lock your main or ‘top’ categories, so your visitors can only post into the sub-categories of those top ones.
As a quick example, you might want to prevent users posting Ads under ‘Motorcycles’ and force them to select a sub-category, such as Cruiser, Sports Bike, Touring, Dual Sport, Scooter etc.
If they select ‘Cruiser’ you might want to also restrict posting in the Cruiser category and perhaps force them to select a brand, such as Honda or Harley Davidson. This helps both human visitors and search-engine crawlers to easily find things!
Note: when your shiny new site is still growing you might want to ALLOW users to post in high-level categories at first, and then use this extension to restrict them to sub-categories later as your business grows.