1. Develop a fantastic new platform/product/ecosystem that people wanted to buy (using local talent)
2. Support Android on some or most of their devices
3. Reinvigorate their current platform somehow (new features, reduced costs, new apps, developer incentives, etc)
Keep in mind, MS's product in this space is widely considered less than successful. The only thing MS has going for it is cash in the bank, but so do Google and Apple and Nokia.
How many people walk into the store thinking "I'd like to sign up to buy a Windows phone"? How is this going to benefit Nokia again?