General Cooling System Question

I understand the damper system to force air through the heater cores for heat and the A/C evaporater for cool and through nothing, for air only. Common sense suggests that coolant normally flows through the heater cores at all times since a valve system would be too costly and prone to failure and overly complicated, but why does everything I read about flushing and refilling the cooling system always suggest running the fans at high speed. It really shouldn't matter, unless the effect of the cooling matters, which I doubt.

Any ideas?

Well common sense did not prevail - or it wasn't so common. I did a bit of reading and apparently it is common to use a heater control valve to control the flow through the heater cores so i suppose I answered my own question. This makes the damper system simpler and allows the AC to work with the defrost.