Focussing on the Magento store, the defer parsing of Javascript helps improve the site speed. Usually, the Javascript is placed between the header tags and the code is implemented from top to bottom when a website loads. This results in loading of all the JS taking time which is not necessary at the point of time.

The defer parsing of Javascript in Magento enables loading of HTML and CSS scripts before the JS. This is enabled by replacing the Javascript from the header to footer tags. It results in the site speed appearing to be faster for the user!

For more info: https://meetanshi.com/blog/defer-parsing-of-javascript-in-magento/