“It's a great move forward without compromising what Rails stands for and its overall integrity—to give you the option to choose where you want to go while still maintaining the convention of configuration.” -David Kimura
Yarn Support in Rails
It can also be used to maintain and update the different versions. Overall, having Yarn in Rails 5 provides more convenience than looking for the source outside and pulling it in. Yarn vs NPM
On Charles' end, what he knows is that NPM includes a dependency management. Although both Charles and David don't agree with everything they've put up, the Rails core team has never failed to provide solid and good reasons for their releases. Convenience with Webpack
The Webpack support also makes things convenient. Charles loved it when Rails adopted copy script, but felt he had better options.
As he moved away, he found it difficult to integrate his options. With Webpack, he can now pull up a plugin and simply ensure Webpack does its job.