Building JavaScript assets offline, before deploying to the web server

I have always wondered why I have to run yarn build scripts online on Heroku / Netlify / Now.sh; which takes time, does not guarantee to succeed and HTML / JavaScript / CSS are cross platform anyway.

Long time ago, I have found gh-pages and wrote a blog about it -- https://www.polvcode.dev/post/2019/08/gh-pages -- which is mostly about publishing to GitHub Pages, and Heroku.

However, there are problems with GitHub Pages; namely, new pages doesn't always update promptly after repeated deployment. There are these kind of problems in Heroku / Netlify / Now.sh.

Lately, I have one another way for Netlify, which is Netlify CLI deploy, so I just pust "deploy": "yarn build && netlify deploy --dir dist" in my package.json. Netlify also offers a way to test a real website online (with hash, so that the name doesn't collide).

So, back to the real question, why do I have to run build script for static JavaScript / CSS / HTML assets online, again?