The team-up promises to greatly simplify voice control in the home. You could issue commands to an Echo speaker that take advantage of Cortana's access to Office- and Windows-specific features, while you could use Cortana to access the many, many skills Alexa offers. In short: instead of having to remember which AI helper to use at any given moment, you could settle on one.

It's also a defensive measure. Both Amazon and Microsoft have to worry about competing platforms. Google is the most direct threat, since it allows Assistant on third-party hardware, but the two have to consider Apple as well -- after all, you're less likely to consider an Echo or a Cortana speaker if you buy a HomePod. Theoretically, this lets Amazon and Microsoft focus on their largest threats instead of trying to take each other down.