Beats have a huge following among youth and teens. Apple could be using this as a gateway for wearable tech to the demographic. They could be incorporating Apple tech into Beats headphones.
Also, don't forget about Apples patent in regard to their new headphones which contain bio-metric sensors, etc. Branding these under the Beats name could increase sales significantly over the selling under the Apple brand name. These headphone won't be the freebies you get with an iDevice and Apple may have a harder time selling/marketing them under their name.
If Jimmy Iovine, becoming a special adviser, is also part of this deal, that also carries significant weight. He has huge ties to the music industry as does Dr Dre. Also, there was rumors about Apple expanding the iTunes store to provide higher resolution, almost studio master, versions of its music offerings. Jimmy could be instrumental here as well. due to streaming services, Music sales have been declining. It could be that Apple has a way of turning that around and beats/Jimmy is instrumental in that regard. Music is a passion of Apple.
Another possibility is that Beats has many subscribers for their service. Remember its a pay for service, unlike iTunes Radio. If contracts are similar or transferable, Apple can then provide that service, probably at a much cheaper cost, back end wise, then Beats pays now. Apple then adds a new service with recurring monthly revenue.
One other hypothetical. What if this is masked as a buyout but is also some sort of payment for services rendered? It stated in January 2013, Jimmy Iovie met with Apple executives to pitch a subscription music service. Jimmy had pitched this back to Steve Jobs as early as 2003. As we know, the music labels were playing hard with Apple on contract negotiations etc. As we have seen in the past, the music labels seem to offer some more favorable terms to Apple competitors with direct music sales, ala Amazon and music deals, etc. So what if Apple played a great game of chess in this regard. Is it a coincidence that Beats waited until Apple released iTunes radio, even though they were readying a music subscription for years. It wasn't that they waited until it was released but way after Apple secured a foothold. Beats then secures more favorable terms for a music subscription service then Apple could have secured on its own. Maybe this is part of a music service Apple wanted to offer but couldn't get the music labels to agree on? Beats's service seems more like a Spotify competitor and the rumors way back when were that Apple was looking to offer a service along the same lines. Apple also fought hard to keep Spotify out off the US. Thus, Iovine creates this music service under the Beats moniker and gets contracts written whose terms do not end, if the company is acquired. Again this is conjecture but what if? Apple then picks up and rolls in a music terms they wanted and all done using Beats as the pawn. Hmmmm.