SQL Server, connecting without windows authentication?
Windows Authentication is always available, SQL Server Authentication is only available if Mixed Mode is enabled.


  During setup, you must select an authentication mode for the Database Engine. There are two possible modes: Windows Authentication mode and mixed mode.


So:

For an instance using Windows Authentication mode, connections can only be established using Windows Authentication.

For an instance using Mixed Mode, connections may be made using either Windows Authentication or SQL Server Authentication.

In other words, Windows Authentication being always available does not mean it is always used. A company may choose to use only SQL Server Authentication, even if Windows Authentication cannot be disabled (it will remain enabled but unused).