Creating multiple styles of your template allows you to use different template settings on various pages across your site. For example if your template allows you to set a background image you can have one style for your homepage with a background image set while having another style with just a plain background colour throughout the rest of your site.

In Joomla creating multiple styles is very easy. Simply navigate to your templates parameters (Extensions ->Template Manager -> J51-[YourTemplate]) and click the Save as Copy option.  Make the changes you wish to make to this duplicate style you have just created. You can then assign this style to the menu items of your choice via the Menu Assignment tab. 

Multiple template styles can also be used when working with languages. Once you have created your new style you can then set it to appear with a particular language using the Language dropdown. This is great if you want to use a different logo/slogan depending  on the language.