I might or might not be a good frontend developer, but I care a lot about writing efficient and maintainable code. The first rule of being a "good" frontend dev is to learn all the libraries there are and ever will be. Nobody cares how you bake it, what you bake is all that matters. Who would write a 100 line JS script when you may just offload the 95 to a library? :)
1. Organized workflow
When you are working on a big project with several developers, it's easy to write Spaghetti codes. You may not realize the impact a mere JS library creates on a codebase, but we are not talking about one libary here. When you get to work on a 5 years old codebase with 7 versions of JQuery, 3 versions of Leaflet and 2 versions of D3.js, you will know what I am talking about.
2. On-site cache
3. Off-site cache
Public CDNs are everywhere these days. From Facebook to WordPress.com to NYTimes and Bloomberg, the hype is real!
Chances are, if you are using a popular library - your visitor's browser already has a version of that cached from an earlier visit. If you use a hosting service for your libraries, your visitor browser will load that library from the disk cache. It will result in faster page load time, and most importantly - a happier userbase. :)