Extra Billable Items are additional billable items that can be quoted alongside your vehicles. That being said, there is no restriction to block you from adding vehicles with flatrates as “Extra Billable Items”, if you have such preferences.
These items can be billed as “hourly” or “flatrate” and so it allows you to define items like a tour guide (hourly) or Champagne Bottle (flatrate) that will be available for system users to include in quotes. This is what differentiates them from the vehicles and why they are handled separately.