A big reason is likely so they can place ads in your feeds while using the app and avoid ad blocking extensions, push notifications, etc.
For mobile usage I use the third party app, Apollo[1], highly recommended.
For desktop I use a couple of third party extensions to improve the experience & normalize the UI, Old Reddit Redirect [2], RES [3]. I then disable subreddit css and enable dark mode. Any subreddit I visit looks exactly the same as any other, it really allows you to focus on reading content.
- https://apolloapp.io/ [1]
- https://chrome.google.com/webstore/detail/old-reddit-redirec... [2]
- https://chrome.google.com/webstore/detail/reddit-enhancement... [3]