If we’re appropriately representing the open source community, you may be inclined to do everything in the public. But what if you work at a company on proprietary code? What if you’re starting a new company where you plan on selling software? Or, what if you’re a student, and you’re working on a group coding assignment? These are examples of when it may be appropriate to work in a private, collaborative environment, when it may be appropriate to inner-source your code.
Inner-sourcing is really just a play on words with open source. It implies that you will use the same (or similar) strategies to collaborative code writing as open source, but you will do it on private repositories.
Source: Guthals Sarah, Haack Phil (2019), GitHub for Dummies, Wiley.