The water heater bypass lets you close off the WH from the rest of the fresh water system so you don't have to use as much antifreeze to winterize. If you don't have a bypass you'll be filling the WH with antifreeze -- 6 gallons -- along with all the water lines and fixtures. By using the bypass, you can drain the WH and use about a gallon, or less, to completely winterize.

John

Also makes blowing out the lines easier since you don't have that large 6 gallon space to do. 

Sent from my Android using Tapatalk.