I haven't found a clear website that I could share with non-profit organizations or companies, about how their opening of data can be used by the community for transparency, marketing, good PR, building a community, 3rd party apps, etc.
The challenge is that opening data is done for different reasons by each stakeholder, so there is no one-size-fits-all answer to the question "why open data?".
This resource from OKFN very much focuses on governments: https://okfn.org/opendata/why-open-data/
This question focuses on governments as well: What are good examples of how open data is driving community development?
But are there more general examples of websites or documents that could appeal to other potential data-sharers such as companies?