If your goal is to do something fun and easy, then jQuery or even vanilla JS is fine. If, however, your goal is to make a full fledged single page application,

If your goal is to do something fun and easy, then jQuery or even vanilla JS is fine. If, however, your goal is to make a full fledged single page application, you'll find yourself running into a lot of solved problems. You'll definitely want to modularize your code by putting it into different files. Soon, you'll find that you're spending a lot of time figuring out what order to load your scripts in your main JavaScript file. That's what Browserify, CommonJS, etc. are for. You'll get sick of recompiling your files and refreshing the page. Or maybe you want to write ES6 code but have it used in browsers that only support ES5. Or you might want to do something simple like concat your files when deploying. That's where the Grunt/Gulp/WebPack build tools come into play. And so on. These tools are massively popular because they solve real pain points for developers working on large scale front end applications.