While I'm sure it's more useful with multiple numbers, I've gotten a lot out of it having just one number. It gives me a second number that I can give out to folks who I don't want to give out my primary number to.

It gives you much more flexibility: you can block certain numbers, forward to calls to voicemail, forward to other phones, record calls, make calls & listen to voicemails online, etc. There's plenty of utility even if you have a single phone.