Personification of the assistant allows a more intimate experience for those not of the computer science inclination. Humans know how to talk to other humans, most are still too unfamiliar with speaking only to "a computer program at a data center somewhere" to get any real use out of it.

Anecdotal, but I have an aunt who hates computers but loves the Echo because of Alexa. By giving the assistant a name and speaking to it conversationally, she was able to "trust" it much faster than many other technologies.