The key is needed to decrypt the messages in WhatsApp. Without the key, the software will display random characters that are not human readable.
Luckily, there is an application that still works even to this day. It's called WhatsApp Key/DB Extractor and it's available for download from the XDA Forums. Access the forums, download the app, install it and extract the key the add it to the WhatsApp Viewer and read the conversations.
