As a matter of personal choice, due to specific work requirements or to endorse company-wide settings, you may want to customize application. For example, you can create and customize toolbars and use them in the same way as default toolbars. You can set the GUI profile (look and feel of the software) of the program that best suits an industrial design work or set it more productive for a machine design work and so on.
By doing the customization you fine tune application and make it look and work the way you want it to be. There are many ways to perform customization. One easy way is to use the Settings Wizard that is shipped with the program. Or, you can always manually set each and every detail for the customization using the system / document properties.