Why exactly does Microsoft care about search?

Microsoft is a software company. Sure, it has made forays into hardware (keyboards, mice, joysticks, and the ever-semi-popular Xbox), on-line services (the ever-semi-popular MSN.com) and information delivery (the ever semi-popular MSNBC).

What pays the bills is the sale of software licenses. Microsoft is a software company.

So why Bing.com and the Yahoo! acquisition-turned-joint-venture?