I imagine this to be like an app store, but instead of apps, there are dashboard parts that users can download to their own dashboards should they wish.

Whilst we can share dashboards between users, I'd argue that unless you've seen another user's dashboard and found a part you'd like, you're not necessarily going to know what's possible / available. A central library that admin users can manage would mean consistent dashboards across your user groups and promote self-service.