Nokia already had a large stake in Symbian, the entity that produces the Operating System (OS) for its phones. Now, the company bought the rest of the shares for $410M and announced that phone makers would not have to pay to use the Symbian OS anymore. By doing this, Nokia hopes to boost the market share of the Symbian OS. Just like Google with its Android mobile platform, Nokia is trying to gain Mindshare and influence in a world where software is more and more where the value is, because it defines the user experience more than the hardware. As an example, just compare the Samsung OMNIA and the iPhone 3G, similar specifications, but very different user experience.

Also, Nokia has invested a lot of efforts in developing applications for Symbian OS, and wants to make sure that its investment stays relevant as the company goes from phone maker to platform and content provider and distributor. When you control the platform, you can shape the industry to your own image  ask Microsoft, they would know something about it.