If you plan to use water as your chill source and keep it above its freezing point you would not require glycol in your system. In fact, there would be no additional benefit other than potentially warding off some contaminant growth within your chill water.
The benefits of glycol for commercial breweries or advanced users are a few fold:
1. Propylene Glycol is a food-grade antifreeze, and with the proper mixture (one-third glycol, to two-thirds distilled water), can lower the freezing point of water significantly down to 5-7 degrees Fahrenheit. This is important when evaporator coils are in direct contact with the chill source because these coils can easily reach 10-15 degrees even when a bath temperature is set to 25 degrees. If the chill source were to freeze, the evaporator coil could rupture, which would significantly damage the system.
2. Glycol adds additional delta between the bath temp and target cellaring temps. For example, if you want to crash cool beer down to 34-35 degrees, you require a chill source that is at least 10 degrees lower than the target to ultimately reach the destination. Glycol also inhibits the growth of algae or other contaminants within a closed system.
Keep in mind that by running plain water through your system at the edge of freezing, you will only ever reach roughly 45 degrees within the fermenter. Furthermore, depending on the liquid volume you are attempting to cool and the means you are using to cool it, may not have the recovery rate to overcome the temp loss. A great example of this is a reservoir placed inside a keezer, since the median for heat transfer is simply the air within the keezer. This is not as efficient when compared to an evaporator coil submerged directly in glycol.
For users without the need to consistently hold lager temps or crash cool, water works great. Yet for more advanced cellaring activities, we suggest glycol.