A couple of reasons come to mind.
More programmers knowing about Swift directly benefits the Apple ecosystem as it means more people available to hire to write software for Apple products, more people who may be tempted to produce Apple-specific versions of software written in Swift, and so on. There is also a halo effect which is less easily quantifiable (e.g. someone suggests using Swift for part of a system at their business, because they happen to know it / have a project written in it which fits the need, etc).
Because Apple is falling behind as a corporate development platform.
The most popular "open" platform for desktop development until years ago was Java. Then Oracle bought Sun and just totally destroyed it. So Microsoft got smart, moved fast and ported their platform .Net to other operating systems and open-sourced it. With that move MS hopes to remain the king on corporate development and increase their presence on cloud computing.
Apple is trying to catch up. But it may be too little, too late.
But there is good news on that front.
People who program for the iPhone and OSX also develop for other platforms (linux, android, windows, etc). Open sourcing Swift means you can develop apps for multiple platforms at once.
In addition to the client side development, most apps also require a server architecture -- this lets you develop a server in linux that communicates with a client on the iphone, both speaking the same language.
Open sourcing swift means developers will be more likely to start new projects in swift, knowing that it is compatible across multiple platforms.
More swift developers means more iOS developers, and at the pointy end of the App Store, higher quality apps.
The most important thing is it gives developers the confidence, that if they learn swift it won't disappear over night.