What the WMF does for us is, among other things: allow us to use their trademarks, provide the servers & technical support that run the projects, develop and support MediaWiki, provide research and some support in specialized areas, and fund development of the movement globally. That is why we give them most of our income.
The relationship between the WMF and the chapter is set out in the various agreements between us, and was also summarized in the submissions last year to the Chiraty Commission; one of their big concerns was our independence.
John