Microsoft has made many acquisitions for many reasons over its history - 122 to date, according to the list maintained at the Wikipedia. Almost 100 of these have been consummated in the last decade, as the company that triumphed in operating system and office productivity software has sought (often unsuccessfully) to achieve similar success in other domains. Other purchases have demonstrated pragmatic "build versus buy" decisions, serving to add functionalities to products that needed them more quickly and efficiently than in house efforts could achieve.
In its earlier days, Microsoft was much more likely to mimic the products of other companies rather than buy them, in part reflecting its engineering-driven culture, and in part its hardball approach to competition. When it did add features this way, it invariably added them for free into its existing products to make them more desirable. The result was often to drive the originators of those features out of the marketplace, since who would buy what they could get for free? Sometimes, the motivation was more desperate, as with the crash development, and bundling, of Internet Explorer in Window, when Netscape threatened to open a critical breach in Microsoft's control personal computing.
If that sounds vaguely familiar, it should, since Google is following the same course, albeit in a kinder, gentler way, as it adds service upon service, all for free, and all in the service of racking up more and more ad revenues. That's disturbing, because when your goal is ad revenues and not great technology, you may not necessarily produce great technology. But as Google's dominance continues to grow, who will be able to credibly compete against it in those technologies, to ensure that innovation continues?