Why to use namespaces? What is the problem with using one default namespace?

When using the default namespace alone, it becomes hard over time to get an overview of all the applications you manage in your cluster. Namespaces make it easier to organize the applications into groups that makes sense, like a namespace of all the monitoring applications and a namespace for all the security applications, etc.

Namespaces can also be useful for managing Blue/Green environments where each namespace can include a different version of an app and also share resources that are in other namespaces (namespaces like logging, monitoring, etc.).

Another use case for namespaces is one cluster, multiple teams. When multiple teams use the same cluster, they might end up stepping on each others toes. For example if they end up creating an app with the same name it means one of the teams overridden the app of the other team because there can't be too apps in Kubernetes with the same name (in the same namespace).