During the boil a lot of water is evaporated off. I have a 30L boiler and generally end up with around 22L in the fermentation vessel. After racking off from the fermentation and leaving the trub behind I can end up with as little as 19L worth of beer.
To increase the amount of beer at the end or to intentionally reduce the ABV I occasionally add water (same sterile water used for mashing) to the fermentation vessel just before pitching the yeast.
What are the effects of adding water at this stage? Pros vs cons.
Follow up question. I intend to upgrade my equipment in the future. I have 30L kegs I use in my kegerator so 30L is the target batch size.
What is the difference between the above method of adding water to get up to 30L vs having a large enough boiler which can boil down to 30L? Is one better than the other, if so, why?