Here are some articles I found while reading about the differences. If anyone has thoughts on them post a comment below.
Rollup, on the other hand, came up with the idea of using standardise format (i.e. ES2015 module format) to write your code and tries to get really small builds with the help of dead code elimination.
Parcel tries to give you “blazing fast” bundling as it uses multiple worker processes to ensure that the compilation process is executed in parallel on multiple cores without the need of any configuration.
By now, hopefully it’s clear why both tools coexist and support each other — they serve different purposes. The tl;dr is this:
Use webpack for apps, and Rollup for libraries
That’s not a hard and fast rule — lots of sites and apps are built with Rollup, and lots of libraries are built with webpack. But it’s a good rule of thumb.
If you need code-splitting, or you have lots of static assets, or you’re building something with lots of CommonJS dependencies, Webpack is a better choice. If your codebase is ES2015 modules and you’re making something to be used by other people, you probably want Rollup.