In a traditional client | vendor relationship the "risk" involved in the project is shared by both. The client pays whole or in part for the service provided; reducing the risk taken by the vendor... The vendor (you) provides the service with the understanding that the fee paid is a flat rate, the rate generally will not increase or decrease. This fixed price reduces the risk for the client, they know exactly how much money they are risking against how much they hope to gain from completion of the project.