@DeathByVisualStudio: It's the same as usual. engineering expertise, IP, manufacturing capacity. Nokia had no real strategy aside from shotgunning out dozens of models in confusing patterns. This likely made for some organizational bloat, because Microsoft seems to be intent on still releasing 6 devices per year with 7,800 fewer people.
MS bought themselves some degree of 'legitimacy' for Windows-Phone with Nokia and the Lumia brand. If they hadn't have done it, Nokia would have gone Android, and that opportunity would have been lost.
However, having done that, there is no need for MS to stay in the business of actually making phones, just to maintain the Lumia product line - they can outsource that in the same way that they outsource the production of Surface, and rationalise the number of products they need, because they don't need to cover costs of mfg capacity that is greater than they require.