It can be useful to run your own package server even if you're not creating your own packages. You can use this to keep a curated list of packages that your team finds commonly useful / has audited / has had the license blessed by legal / ???, or to be productive during an internet outage, slowdown, or lockdown, etc.

If there is no Internet, I'm going home. You can't seriously expect me to work without StackOverflow

It's also nice to not have your CI/deploys depend on third-party sites being up.

There's plenty I can work on without SO - that large backlog of things to document, for example. And just because your dev machines are on a locked down network doesn't mean you have no internet.

If perforce is down, however, I'm starting a riot ;)