This is a matter of marketing, costs and strategies :
marketing : if there were no incentive to go through this kind of broker, they wouldn’t attract any customer and they wouldn’t exist.
costs : this is usually just about a website. They don’t own any car inventory, any retail store and have very few employees so the costs are slightly less than a car rental company.
strategy : car rental can’t discount themselves everywhere so their strategy is usually to give you a good price through this kind of websites to attract you and build brand loyalty with the hope that next time, you’ll book directly through them. Then, they put regular prices on their own website but you have a lot of ways to get discounts via loyalty cards and newsletter…