[yoyoyoda]

My old ford had one, I removed it, the battery lasted only 2 years instead of the much more average 4 years or greater. Its designed to thermally insulate the battery from the rest of the engine bay and give it a slight cool air source.

Simple stuff really, when charging a battery it produces heat, when discharging a battery it produces heat. When you run an engine it produces LOTS of heat, combine the 3 and you get a recipe for boiling electrolyte away which isn't good for longevity of a battery.

The fact that batteries produce oxygen and hydrogen and acid doesn't even factor into the reason why the battery box is there, the vents are still exposed at the top with or without the battery box that they put into most car engine bays (some cars have them fully enclosed), so the battery is going to be venting those gases into the engine bay anyway, and engine bays generally do not produce naked sparks, usually the sparks are enclosed inside of the engine.

The second reason why the battery box is there is for a slight amount of mechanical vibration insulation to the bottom of the battery, lead acid batteries are made up of semi-fragile lead plates and leads connecting each plate to each other, to form a pack of cells which gives you 12volts nominal per battery.

Without the battery box the battery would be sitting directly onto the metal chassis of the car, absorbing every pothole and all the vibration the engine can put out.