Wordpress Editor - What the "Align Full Width" button does.
Align Full width button is found in the second line of the editor menu, in between the "underline" and the "text colour" button.
It has a stack of horizontal lines as an image. What the lines show you represents the style of spacing that you see when you open a book to read.
The text runs in a line down the left and right margins of the page, thus creating a much neater finish.
[The blog editor here on this editor does not have that button, so I am unable to demonstrate it here].
Please grab a bunch of text, copy and paste into a sample page and view it. Then come back and hit that button and view again.
So, in a nutshell. The reason I would suggest you use it is to make your paragraphs aligned left to right.
First Highlight all the text and hit the button! Happy editing!