Update
This article is dated and while Snowpack was a great option at the time there are now more modern alternatives such as Lerna, Rspack, Turbopack and Vite.
Skip the bundler, run snowpack once, use ES modules and get instant live reloading.
Snowpack exports your node module dependencies as ES modules to a web_modules folder, that your application can then import those dependencies from. Snowpack handles your dependencies it doesn't touch your own code.
As a developer this frees you from having to use a bundler (Webpack, Browserify, Parcel, etc) to make use of those dependencies - providing a more efficient workflow. In contrast to a bundler based workflow that must process your dependencies repeatedly on each change, snowpack only needs to run once to export your dependencies. You can also continue to use your favourite tools like Babel and Typescript.
For your users it also means those dependencies don't have to be downloaded as one big bundle - and instead can be fetched individually. Serving your scripts individually rather than bundled may also improve performance as a result of greater caching ability and potentially faster delivery.
In some cases the performance of serving bundled files will be faster than serving many individual files. In those cases you can then opt to use a bundler as a performance optimization, there's nothing preventing you from doing so. The key here is that adding a bundler is a deliberate choice, rather than something you're forced to do in order to use your node module dependencies. See Snowpack's notes on load performance.
See the official getting started guide here https://www.snowpack.dev/#get-started.