LOE is optional and not obligatory, It is always your best place when u want to explain something that need to be clarified in your application.
Cases vary, every one has his own case and may feel something needs to be clarified.
It is written in your own words in a decent formal format. You can also include any document to support your case.
If you feel your application (Main Required documents) is fulfilling everything and clear enough without explanation, then you have no obligation to write a LOE.
Hope this helps & Good Luck.