Apple is set to make the biggest purchase in its history: A $3 billion-plus deal for Beats, the company that sells high-end headphones and recently launched a streaming music service. The deal, first reported by the Financial Times, isn’t closed yet, according to people familiar with the transaction. But let’s assume it is going to go through, generating a big payday for co-founders Jimmy Iovine and Dr Dre, as well as investors who include the Carlyle private equity firm, Len Blavatnik’s Access and Universal Music Group. Next question: Why is Apple doing it? Apple has more than $150 billion in cash on its books, so it’s easy enough for the company to pull the trigger. But Apple has traditionally kept its M&A activity to smallish pickups of technology companies, not spending billions on established brands. What has changed now?