I don't get Cortana either!
How is it going to work in an office? Even if you choose to use it, Cortana will need to know the person talking to her/it is the right person and not an overly loud person sitting near them or indeed someone going thro' the office saying "Cortana shut down PC".
Even in the home environment I think it will have limited use, maybe we just need to get use to talking to our computers. Again, really needs Cortana to be intelligent enough to accept direction from the correct person. (So someone on voice comms/skype or a TV in room cannot trigger it).
However, for people who have difficulties typing it might be a godsend. So include it, just allow people to disable it.