Many applications will create user accounts in the database that do not own anything. They have privs to query the main application schema, but will never be used to create tables, views, etc.
When you’re browsing the database via the Tree in SQL Developer, you probably won’t want to see these ‘fake’ users.
Just Apply a Filter
Make sure your data dictionary stats are up to date – this extra check for object counts could get expensive, esp if you have thousands of accounts.
Remember, if you want to administer these accounts, use the DBA panel.