I'll second several of the above reasons. I have an airtrap/reservoir for getting air out of the system, providing a convenient fill point, and providing a safety margin of coolant in the case of evaporative loss.
Oh, BTW I have vinyl tubing (yeah, yeah) and I still lose about an ounce every month or two. However, my airtrap top isn't a great seal. So that's probably where I'm losing it. Water gets hot and pushes humidity laden air out. I shut the system down and the airtrap sucks a bit of cooler drier air back down inside. Well, it's a good theory. Hopefully it's not a micron sized leak somewhere else.
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