Every Toyota Radiator is made of hundreds of small channels that are placed vertically in its interior. This is the area the coolant will flow through and how the heat can be effectively dissipated from the liquid medium.
Most Toyota owners do not notice the channels in their radiator core because of the small metal fins that are attached to them. It is these small fins that draw the heat away from the channels so it can be transported to the surrounding air. If the coolant system is flushed annually for the life of the vehicle, the radiator should last the lifetime of the vehicle.
The leading reason for the flushing of the coolant system is to remove the residue build-up that forms in the channels in the Toyota radiator core. This residue is from the dissolved solids in the coolant and water mixture used as the coolant medium in your Toyota. If the residue is permitted to build up in the interior of the core, the channels will become clogged preventing the flow of coolant through it. This will also stop the transfer of heat from the coolant to the fins on the radiator.
As the surface area of the channels in the radiator core is reduced, the ability of the coolant system on your Toyota to remove the heat being generated by the engine running will not be an efficient process. In the long term this will lead to the engine having to operate at a higher temperature than it was designed to run at. This will lead to premature wearing of the seals and gaskets on other engine components causing them to fail or rupture.