What else would it be used for? When you completely separate the two systems, you can use different coolant fluid. For example, I use straight distilled water and a bottle of water wetter for the IC system for most all of the year. The reservoir serves to fill the system, flush the system, check the fill level, check to see if it's flowing well or not, drop a temp. gauge into it after a hot lap if you're in to logging that or not, can be fitted and used to pressurize the IC system to find/verify a leak(hopefully a hose end and not your IC!)