I am by no means mechanically inclined, but I do like to lurk.
The air bleedout would be required if someone drained out fluid from somewhere, capped off the drain and refilled. Or anything else that would lead to air being caught somewhere in the lines, like replacing a coolant line, radiator, plug, etc.
For extra credit, air in coolant lines it would cause your car to overheat and air in brake lines would cause spongy brakes.