Apologies Are Dangerous - S3-E4
Continuity mistake: When driving to their first call that shift (guy stuck between cars), Truck 81 is seen arriving at the scene while it should be still being repaired. When they arrive back at the firehouse, the truck they drive carries the number of the exchange truck they've got (E-..).
Continuity mistake: Brett enters the abandoned factory twice when she is searching for the missing girl.
Answer: Internet sources give many reasons why firefighters do this, but in regards to your question, it's probably because opening walls and ceilings creates better ventilation which allows heat and smoke to escape, giving firefighters better visibility and lowering the temperature.
raywest ★