I use it because Verizon only allows you so many voice mails on their system... I am very busy during the day and at times I can get 20 voice mails backed up.
With google voice I can also re-direct calls based on the time of day, who they are, and so on.
I could not live without it after using it for the last two months. It also allows me to have my voice mail on my phone and not have to pay for it like Verizon charges with Visual voice mail.