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!