Many web applications are set up with default credentials to allow accessing it after installation.
Many platforms provide lists of default credentials for a wide variety of web applications. Such an example is the web database maintained by CIRT.net. For instance, if we identified a Cisco device during a penetration test, we can search the database for default credentials for Cisco devices.
Further resources include SecLists Default Credentials as well as the SCADA GitHub repository which contains a list of default passwords for a variety of different vendors.