It's the default and you can opt-out.
If you're interested in front-end performance, having a webpack build pipeline for your frontend assets is quite useful, especially if you want to leverage libraries like react or vue and build interactive reusable components, or leverage existing ones.
Tree shaking, minification, chunking, splitting, etc. will help you deliver a faster initial page load, which is a better overall experience and is also factor in search ranking for your public pages.
Front-end engineering has changed because users expect interactive experiences. Rails has an opinionated way to solve this differently using turbolinks and Rails UJS. A very significant proportion of folks don't want to go that route, enough for it to become the default.