Lets look at an example of a Universal app, Huetro for Hue by Neils Laute, Neils has supported WP and Windows but now he only has to make one app that runs on the one platform, so he can concentrate on just one thing saving time and money, for me I only need download one app on whatever platform I want it on, and I only have to show my wife one app and that saves me so much time and effort you wouldn't believe it.
But for other developers who have been on one platform or the other, now just they just have the one, for all the people who reckon PC users can just use a web login, now they have a reason to think of apps, because not all PC users use desktops, some of us use phones, some small tablets, some use transformers and there are even one or two traditional PC's about, the point is Windows isn't just a dying OS stuck on PC's, it's a young upstart that can run on almost anything including fridges.
Developers will want to be involved because this is going somewhere now.