The application allows you to have multiple WhatsApp accounts on the same device. For instance, if you use WhatsApp on your phone, you can add an account on your tablet. You will not need to add your contacts manually. 

Moreover, it is possible to have two WhatsApp accounts with the same phone number, but on different devices. In this way, you can send and receive messages in both your primary and secondary accounts at the same time.

In order to use the Whats Web app, you need to make sure that the main account is not deactivated.