I keep music on my old phones. Email and other such stuff that works with wifi too. Good for throwing in the Jeep, or taking on a plane. Some will even function as a GPS without having a phone contract.
Ya never know when your primary will take a sh!t, and you need something to get you running real quick.
eta.. My phone is company provided, so I'm kinda stuck with the devil (Apple). I have an Iphone 10 now. But, between the 3 I've had, 5S, 7, 10, the 5S was probably my favorite because of it's size. I really dislike that they did away with the home button, and headphone jack on the newer ones. Only reason I upgraded was because it was short on memory..