How are formatting marks useful when editing document?

It is easier to edit document when formatting marks are visible. It also shows you how things have been laid out. It can highlight errors in the layout and these can be corrected. For example, people often use spaces to push text to the centre of a line. With the formatting marks off you won't know have they done that or used the alignment settings, which is the proper way. You can also see things like where people have used tabs or not.

As a general rule, you should not use the spacebar to align things as it can cause problems. For example, if you change the size of the font, or the margins, things won't always stay where they are. If the font was increased, that will have the effect of making each space bigger, so something that was in the middle, will be pushed further across. If you use the alignment options to put it into the middle, then adjusting sizes of fonts or margins will not put things out of place. So by having the formatting marks on, you can see those kinds of things and fix them by using the correct kind of formatting to position things.