You'll have to ask Jed Watson (one of the owners of Thinkmill) for a specific answer to that! I can tell you what it looks like from my side:
Thinkmill, as a company, is much faster at producing value for clients due to open source. We use lots of OSS (React, Node, MongoDB,..) and without that we couldn't do what we do. Investing in a developer to work full time on open source is a way of giving back to something we've gotten a lot out of. (next to every employee working ~25% of their time on open source)
Also, I build tools that help Thinkmill do their job faster and easier. styled-components is a great example that'll help a lot when creating cross-platform applications, and allowing Keystone users (which includes our clients) to theme their applications!