A: I believe they are collaborating on Tizen development because they want an alternative to Android and because Bada was not good enough to meet that goal. Tizen’s roots include contributions from Intel, Nokia and several Japanese companies so it’s not Samsung’s OS per se. Historically it was the “open” alternative operating system for embedded and mobile systems. See https://github.com/kumadasu/tizen-history/blob/master/tizen-history.pdf for a historic perspective.
In some ways Tizen seems to be a blend of several pieces of code including proprietary Samsung user interfaces. Samsung is presumably interested in having a unique, differentiated experience. This is something every device maker seeks. Put another way, if they did not seek this then it’s unlikely that they would be profitable which would also imply that they would not invest in marketing and development of products. As Samsung invests both in marketing and development it can be concluded that they seek to offer a unique value proposition and, in today’s market, that means a unique experience.
Does it make sense to create a new mobile OS when it has had so much success with Android?
Samsung would argue that the success it’s had is not due to Android but to its products. Arguably they are right because if Android were the valuable component in a phone then buyers would buy the absolute cheapest device that runs Android regardless of brand. That is not the case. People still seek out a particular brand of phone because of the promise it offers. Consumers have been buying more Galaxies than no-name Android phones.