Hmm. If a website is using Bootstrap, jQuery is included.. Considering this, a minified version of jQuery is 82kb (can be compressed even more if you are chucking out unnecessary code in library). Having said this, lets say there is a third party JS library that is only available in UMD way. In case of payment or mission critical applications users should not have access to this script unless they go on that page/service. So for such cases we can load it from componentDidMount(..). There can be other uses too.