What is Google Voice?

What does it do?

Users can set up a new phone number or use an existing phone number to set up a voicemail box through Google.  Users can view voicemails as text messages, text online or listen to them from their phones.  Users can set up one number to ring on multiple phone lines.  They can also set up a “Do not disturb” rule, which sends calls directly to voicemail.  Users can also send/receive text messages.

Why is this useful?