Agencies that get paid by the hour make more money the longer a project takes.
In the hourly model, the agency profitability is highest when they hire cheaper talent that takes longer to do the work, leaving the client to pay for the inefficiencies of the agency.
In the points model, agencies must hire top talent who deliver results in the most optimized process possible, making the agency focus on optimization and utilization of resources to achieve profitability.
Do you want to pay for that junior designer to get their sea legs?
No. You want to pay for a great finished product.