Everybody is different. I've been carrying a phone in my pocket for years, long before I got phone service. I used to use them for audiobooks, games and videos. I've never had an accident that damaged one. I've never used a case. Wait, once about 5 or 6 years ago, while walking into the bathroom reading with it in my hand I stumbled and it flew into the toilet and died a horrible death! Fortunately the manufacturer replaced it even though I told them what happened.
My landlady, on the other hand, had to replace her Iphone because every time she dropped it it got damaged a little more till it finally broke. She replaced it about 4 months ago with a Galaxy S8 which she's since dropped and damaged at least 3 or 4 times. it now has a cracked screen which is getting worse and it's probably not going to last much longer.
The Iphone was in one of those damage proof cases. The Galaxy wasn't. It's dying a little faster but neither lasted very long.
I just got a case for my new Nook. I keep my Kindles and Kobos in cases. But I always have a Kobo Aura handy without a case because it fits so nicely in my pocket. Lately I've used my phone instead but that Kobo is probably 3 or 4 years old now and it's been all over the place with me many times and it's still like new.
The benefit of a case is that it makes the device easier to hold and helps me worry less about it. The secret to not breaking these things is very simple: don't drop them. Don't set things down on them. Don't stumble while walking toward the toilet!
Barry