that tank is an expansion reservoir, when the engine runs and the temperature rises to normal it creates pressure, this pressure is regulated by the radiator cap.
The cap has a big spring in it, that big spring is lifted up by this pressure and then coolant/water/liquid is pushed into that plastic tank (thats why its max/full line is actually at half point) - hot running engine will have the coolant level above max/full in that tank.
Once you shut down the engine it cools down and the liquid in the engine contracts in size - this creates a vacuum, this vacuum then proceeds to pull down the little vacuum hat in the radiator cap and then proceeds to suck coolant from the plastic tank into the radiator - ergo topping it up.
In a normally operating system you only need to make sure the level in that tank is at max or between min and max. If the tank is completely empty or completely FULL - then you have a problem and need to open the radiator cap to check.