There are numerous reasons to have your own
noise-canceling headphones when you travel, and this includes all those times when someone near you is listening to their device on full blast. You may also want to watch a movie or listen to music on your phone to pass the time, and having your own headphones can help.
There are definitely airplanes with seatback entertainment, and your airline may offer plug-in headphones you can use in those times. However, it's fairly common for airlines to run out of the "free headphones," necessitating the need for your own pair. If you have wireless headphones, you can even bring along an
AirFly device that lets you use your wireless headphones with a seatback entertainment device.