[hartror]

Exactly.

Many web tool chains involve pulling from external repositories such as npm, bower or even github itself. If one of these are down you cannot deploy your application.

Even better many packaging systems such as npm or setuptools allow you and library maintainers to specify a flexible version numbers for your dependencies. If during the course of your build, test and deploy chain one of these dependencies changes your application could break through no fault of your own. You cannot rely on maintainers to not release breaking changes in minor versions, it happens all the time, intentionally or not.

[Touche]

At least for npm there is a solution for that problem: https://docs.npmjs.com/cli/shrinkwrap