Hi Stew
Your question about pressure is a good one, the purpose obviously of a pressurized coolant system is to raise the boiling point of the coolant and as with modern vehicles this only works to a point when coolant must be released as it expands. Without a tank it is lost with a tank it is captured to be drawn back into the engine.
The coolant recovery tanks, as seen on the left front side of many CMPs is to capture any coolant that is pushed from the radiator as the engine warms up. If the radiator and cooling system is air tight then as the engine cools the fluid in the recovery tank is drawn back into the cooling system. If every thing is working correctly it keeps the radiator completely full of coolant.
The tanks is made so that though the tank is vented the line from the radiator is at the bottom of the tank. The coolant recovery tank even helps condense the coolant out of the steam if the engine blows steam into the tank. There is a vent incorporated into the tank which is suppose vent the air/steam at the top of the tank. This only works marginally well as the photo of the result of a broken fan belt on the highway. In that case the over blew through the tank and out on to the ground.
One other strange problem that I had once was on my HUP was that mud wasps decided to plug the vent from the tank while building a nest. Next time I took the HUP out as the engine came up to operating temp. There was no where for the air in the coolant tank to escape, which turned the system into a pressurized system until a hose blew off.
The tank location up on the side of the nose has the advantage of being up in the air stream but it has the disadvantage of being out there to be hit by almost anything. The tanks on Chevys were relocated down under the fender to protect it.
Hope that explains the why.
PS looks like Grant beat me in responding.
Cheers Phil