Gonzalo García takes a crack at why webpack (not capitalized like npm) exists at all. No particular disagreements here, but here's my crack at it...
We use webpack because we need to import stuff from place;. This is a good pattern. We can use webpack to interpret those statements, as native support for them isn't what it needs to be yet, and it's not clear whether the native version will be smart for performance or not (probably not, at the scope of projects webpack is usually used for).
We use webpack because of npm. Powerful features are a yarn or npm i away and so our projects are loaded with stuff to import.
We use webpack because we're sure it performs fancy magic that results in good performance-related things for our websites. We cross our fingers we have that right, and we've done our part right.
We use webpack because there is a hive mind in this industry and it leads to a lot of us hopping on the trains with the most people on them, and people are hanging out of the windows of the webpack train.