Air can't be pumped. So if you have airtrapped somewhere in the line, the coolant basically freezes where it is when the pump hits the air. This makes filling really, really hard if not setup right.
Are res near or around the pump keeps the air from ever hitting the pump. There are other ways to do this like a double tap (basically two air traps with a valve between them, you close the valve and add water to one which forces the air into the other, open the valve and you have a filled system), but a res is probably the simpliest.
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