It can be an automation thing. With tools like (Github) Codespaces there are places today that have the ability to deploy every random PR branches and do PRs in a full code editor backed with a running deployment that you can browse. Admittedly, that's a lot easier to do when what you are deploying is web-based and automating spinning up new URLs (even just proxy forwarded localhost URLs in the case of Codespaces) is easy and things are a lot harder to automate with for instance mobile apps.