As already explained most of the accounting entries are created while posting documents such as purchase invoices, sales invoices, etc.
The accounting entries which do not match to an existing Document Type need to be posted to the ledger by using a G/L Journal.
Besides, a G/L Journal can be used to create "G/L Item Payments" or payments do not related to orders or invoices.
This feature is very useful while posting an employee payroll to the ledger as the payroll payment can be created at the same time.
Please review the How to manage payroll accounting and payment article for additional information.
Finally, a G/L Journal can also be configured as a template.
This feature allows to create the same journal entries as the ones contained in the G/L Journal set as template.
This feature is also very useful while posting employee payroll for instance.