WhatsApp Web and Desktop give you all the features of the messaging app, with the convenience of a full-size keyboard and screen.
As your WhatsApp account is linked to your phone number, it is not really surprising that it is thought of first and foremost as a mobile app. While this is fine for messaging on the move, when you're sitting at your desktop or laptop, it can be pain to have to switch between devices as you bounce between working or browsing the web, and keeping up with your chats.