From a non-privacy perspective this adds the feature of being able to customize your wake word, which besides being a nice feature on its own also counters Google and Amazon's desire to inject their brands deeper into our psyche by making us say them out loud.

From a privacy perspective, having user control of the wake word prevents Google and Amazon from adding future wake words that could be abused for other ways to track us. For example, Google might get the bright idea to track TV ads by listening for audio in the ads. Or tracking people in your house by making Android phones emmit non-audible chirps. These kind of "features" could be easily introduced at any point in the future by an update to privacy policies that nobody notices.