Nadella was allegedly opposed to purchasing Nokia. He has since said he's changed his mind, but we can't know what he really thinks.
The currently accepted explanation is that MS doesn't want to be an all out hardware manufacturer, but can't get by without doing any hardware at all either. Why? because:
a) MS would otherwise be too far away from consumers and their feedback. Ballmer thinks their lack of a hardware smartphone division is part of the reason why MS missed the mobile revolution. If all the feedback you ever get reaches you only indirectly through OEMs, you'll never really have your finger on the pulse of what consumers want.
b) MS needs at least some Windows devices to be best-in-class products. Most hardware companies aren't too interested in taking on that task. You can't confront companies like Apple if your software is only ever delivered to customers on cheap hardware. Somebody needs to be pushing the envelope of what a combined Microsoft Windows + Hardware package is capable of, and that simply takes to long if you can only get your innovations to market through other OEMs. The recently released band, according to Daniel, also follows that strategy.