It is all about experience.
When you first upload your project in live server, it will be too slow than your local development environment. So, obviously we want to reduce the loading time. How? Simplify, Minify, Compress your code, using front end framework.
When you have frequent updates in your live server or if it is a long term project, you need to maintain your code in an -easy to understand, easy to modify anything- structure for developers, using framework.
At the time of competition, you need to change, trendy designs in your website suitable to the market. You can do it hassle-free with frameworks.