Think of it as a giant auction with multiple brokers.
Multiple brokers of varying ability to get a good price for you, especially since nowadays to brokers you are more the commodity they are selling than the customer.
Unless they let you trade with direct market access for a commission ... then the ability to get a good price is only on you. Dunno how easy that is any more though, after Navinder Sarao played the players I assume it's much harder.