The main reason Apple buys companies is to bring them in-house because the represent a critical component of an Apple product.
With Beats, Apple gets patented speaker technology and streaming audio. Beats may have also had other breakthroughs that no one knows about for all that money. I know that Apple spends every dollar as if it was their last.
This could enhance iTunes and iPhones, AppleTV, iPods, Macs, iPads
Wow... almost every single Apple product could...