Microsoft is a traditional B2B company; they make boring enterprise software and sell it through traditional resellers. As more and more core business functions (like HR managed services) start being sold through these channels, it makes more and more sense for Microsoft to get into it. More importantly, Microsoft already has a sales relationship with nearly every company in the world -- and they can now cram a high-margin talent acquisition / management product down that sales pipe as well.
IMO this is one of those acquisitions that wouldn't have made sense for Apple, Google or Facebook -- their customer base is too consumer-centric. But given Microsoft's heavy enterprise sales base, it absolutely makes sense for them. Nadella has finally gotten Microsoft to realize it's not really competing with Apple, Amazon, Google and Facebook -- its real competition is Oracle, Salesforce and IBM. Viewed through that lens, this is a great purchase.