Microsoft hasn't offered a reason for the new option, but probably doesn't need to, given that many users will find "Cortana" easier to say, just as Amazon's "Alexa" is on supporting devices. Alexa users can also choose an alternative "Echo" invocation, but owners of Apple devices must use the phrase "Hey, Siri" (and its equivalent in other languages) to get the attention of Apple's digital assistant.
Apple has never indicated it will offer a way to customize the "Hey, Siri" invocation – perhaps because it's not technologically viable, given the way Siri listens for the activation phrase via an energy-preserving coprocessor. But the ability to do so could theoretically help users specify which device their request is directed at when multiple Apple devices are present ("Hey, HomePod," for example). Aside from offering a little more personalization, the option could also help device owners prevent accidental or prank requests from other sources, like TV ads, podcasts, and so on.