If you are referring to the one that hooks up to your Freezone jug hose, I always thought that it was used to determine if added refrigerant was going into the system as a gas or liq? Another reasone for it as I have been told is to see what your recovered refrigerant looks like..... Although I cannot say from personal experience, these are just bits of info that have been passed on to me by coworkers.
Anyone that uses this glass care to explain the manufacturers (real) reason for this sight glass use?