From Cognos SDK Installation and Config Guide
"... Install the SDK files from the IBM Cognos 8 SDK CD on every computer where SDK applications are developed, and in the same installation location as IBM Cognos 8. ..."
There is also a sentence which says
"... After you successfully install and configure the SDK, it will enable you to fully automate all IBM Cognos 8 functionality: data modeling, report execution and viewing, authoring, Web application integration, administration, and configuration ..."
I believe some functionality in the core solution of IBM Cognos 8 do not open up for developers until SDK is installed. I believe this has something to do with licensing. You need to purchase SDK license in order to install and then work on it. This way it restricts only certain licenses customers to use the APIs and not everyone.
If this wasn't the case, one could develop SDK apps just by having couple of dlls on one's pc.