It's useful for source auditing. A lot of third parties like to review source code of things on behalf of customers, this makes it a bit more straightforward.
Personally I'm glad it reaffirms Apple's commitment to open source (which has been strong since the early OSX days), and to refute those who claim the OS shipping on Android is functionally more open the iOS (does Google publish the code for their common library that almost every app requires?).