Reserving a username isn’t currently available on WhatsApp Web or Desktop so you need to use your phone to do it. It’s also completely optional – you don’t have to reserve one and WhatsApp will keep working in the same way. Once your username becomes active, people who don’t have your number saved will see your username instead – so people will see your username in group chats. You still need a phone number to create and maintain a WhatsApp account but when someone messages you for the first time using your username, they won’t see your phone number. Reserving your optional username doesn’t take too long – just go to Settings > Account > Username in the app, and you can choose yours.