Imagine you are currently LIVE on Instagram, publishing a YouTube video, or posting an Instagram Story... Then you don't have to send your audience to any links that EXTREMELY limit your reach as they lead to an external website. No, you just say: "Hey just download my app, if you want to reach goal XYZ". That's it. An app is ALWAYS pitchable and within 5 seconds your prospects immediately know what to do.
2. Users don't need to register anywhere
People hate having to register somewhere, then laboriously go to the email inbox, click a link, sign up. When the browser closes, they forget the password and never return to your site, simply because it's too much hassle. With your own app, the prospect has your content available on their phone EVERY TIME, can close the app with one click, and return to it at a later time. Your content is consumed in its entirety much more often than via a classic browser website.
3. App ensures extreme trust
"Wow, he even has his own app now"...
People trust normal links on normal browser pages way less than a cleanly listed app in the App Store and Google Play Store.
4. Recommendations
In which case do you think you will be recommended more often? When someone has to share a link and a complicated sign-up process follows, or when someone just has to tell their friend, "Hey, go download app XYZ, it's really cool content"?