A virtual number is simply a second (or third) line on your cell phone – you can make calls from it using the QuickCall.com app, and you can have people call you on your virtual number.
This is helpful if you want to have a separate business line.
Also, it’s useful to save your customers or friends money if they are calling you from another country. For example, if you live in China but you do business with a lot of USA people, you can get a virtual USA phone number. It’s cheap or free for them to call to your USA number, and you pay a low rate when making calls to them from your number.