Of all the things airline passengers are asked to do before a flight takes off and lands — buckling our seatbelts, stowing our tray tables, returning our seats to their upright positions — opening the airplane window shades seems pretty incidental. Turns out it’s not.

A recent Reddit thread posted to the r/YouShouldKnow subreddit by a flight attendant under the username u/TerraFormer 001 explained that opening the windows before takeoff and landing is actually a matter of safety. The goal is to help the human eye adjust to the outside light to ensure that passengers can evacuate as swiftly and safely as possible in the event of an emergency, without wasting time trying to orient their vision. The poster noted that the cabin crew always turns off the cabin lights during takeoff and landing for the same reason.

According to a 2018 news release from Amsterdam’s Schipol Airport, there’s another explanation for why airplane window shades need to be open during takeoff and landing, the two most dangerous stages of air travel. It allows the cabin crew to see if there are problems with the engines or wings that need to be communicated to the pilot or emergency services, as well as identify which side of the aircraft is safer to disembark from if the plane does need to be evacuated. In the event that emergency services is contacted, the release also points out that the fire service benefits from having the window shades open so that they can see if there’s smoke or fire in the cabin before they enter.

In response to the original Reddit post, one user commented that they recalled being asked to close the airplane window shades to keep the aircraft cool. Others said they’d had similar experiences on flights to destinations such as Las Vegas and Dallas. In response, another Reddit user who identified themselves as a flight attendant explained that this is common practice once the plane has safely landed.

“Once at the gate, if the plane is not staying in ‘full power mode’ it will start to get hot just as a car does,” the commenter wrote. “Then it takes a long time on the ground to cool back down for a new departing flight, so [closing the airplane window shades] after landing helps a bit.”