well with no ventilation the plant will die, it needs to be able to do its gas exchange, over enough time there wont be enough essential gasses to perform the basic cellular functions. However, when people grow, just when you open it up to look at the plant, you circulate the air enough to prevent this. The secondary main purpose of ventilation is its the best way to keep temperatures to a minimum, if the temperature of the air gets too hot the plant wont be able to process cellular exchange, you also cook the plant. Ventilation is good, the only thing that should limit you in ventilation is the amount of sound that is produced from fans.
Ventilation is really necessary for the plant. Air flow allows more CO2 and fresh air to enter the grow area, as well as control over temperature, humidity and preventing pests/molds to form. It is very essential for a successful grow...
Ventilation gives the plants fresh air that they need, controls temps, and is a needed component of odor control.
The same thing that happens to you if I put you in a room with no air. When your indoor growing you need to recreate basically conditions that are outside. Thats why so many grows lead to fail and its the most fucking easy thing to grow ever.