First of all, you can get all your boarding passes right on your phone. They are easy to pull up on the fly and to scan when going through security or getting on the plane. No paper passes to fumble with.
Second, you can check in online usually in less than 30 seconds. You get your boarding passes instantly and directly to your phone. If you have an iPhone, you can save the pass to your wallet which makes it easy to access in case you have any issues.
You also get easy notifications right to your phone. When I went through the bumps and delays this past trip as soon as the airline gave me my new flights, they were on my phone in minutes. No tracking which paper ticket I had where and when.
I don’t know why anyone wouldn’t want to do this.
Although I have disliked my United flying experiences, I was impressed with their app and the ability to see where my plane was coming from and it was easy to track during delays. Each airline app varies slightly but it makes travel easy and everything is in my phone. You can manage all your air miles right from the app as well.
So, next time you fly download the app!