Acquiring Yahoo would bring Apple amazing attention and traffic, social sites Apple could control, high-quality destinations for the high-quality pictures and videos that Apple’s high-quality products enable people to generate, better social signals and the ability to make more money from fewer ads. It’s also worth noting that Yahoo has been working for some time on the (time consuming) process of building a content-creation engine, having hired broadcasters, writers and editors from major mainstream media outlets, and has also been working on the poaching of YouTube stars for a possible upcoming alternative to YouTube. All this would be a nice and exclusive addition to Apple’s offering in the streaming TV space in an increasingly hot space.
I can't really see why Apple need to own Yahoo though. They can still use all their services and integrate them into iOS without owning them. Yes to secure the future maybe, but why not buy them when they really have to - when they are going down? There is no need for that at the moment.