Why would I use a Webpack?

Question: I am having a hard time grasping why I would use Webpack, and what it is really for. […] It seems like I can't just drop a script in my page anymore. Instead of focusing just on the particularities of Webpack, I think it's more interesting and useful to extend the question to all JavaScript Packers. Frequently called bundlers, there are a number of tools, such as Webpack, Rollup, Browser

[ledsun]

これこれ。これが私が技術書典向けにバンドラーパターンとして書きたかった内容です。