Rinse Aid should perhaps be called a drying aid as that's its main purpose. It is added to a special compartment in the machine and a very small amount is released during the final hot rinse cycle, to reduce the formation of water droplets on crockery etc and so ensures that the water dries away properly and evenly to avoid leaving smears. They contain surfactants which lower the surface tension of the water, so water tends to form sheets, rather than droplets.
Most dishwashers enable you to adjust the amount of rinse aid that is used towards the end of the cycle. If you have smears on your glass, you may not be using enough. However, if you use too much you are wasting money and there's a possibility you may be able to smell or taste the excess.
You can also use White Vinegar as a rinse aid. This may work out cheaper and removes water deposits as it is a mild acid, so removes the mineral deposits. It can also help keep the dishwasher free of limescale.