What does a coolant reservoir do?
Furthermore, why is the coolant reservoir empty?
Constantly low on coolant If the reservoir cracks or develops any small leaks it can cause the coolant it is storing to leak or evaporate at a slow rate. The leaks may be small enough that they may not be obvious to the driver, however, over time they will cause the reservoir to empty out.
Regarding this, what is the purpose of a coolant overflow tank?
A radiator overflow tank collects the expanding coolant that is heated by the engine and recycles it back into the coolant system once it loses enough heat. The radiator overflow tank works in conjunction with the radiator cap to protect the engine and prevent coolant loss due to overflow.
If you drive without the coolant cap for any length of time when the engine gets up to temp it will boil over out of the reservoir. You will lose at least half of the coolant in the system if you drive like that for a few days.