There may be more to it than that. If you click the sign on the toolbar you
can turn on viewing all non-printing characters. But when you toggle it off
it may not hide them all. It depends on which checkboxes you have selected
in Tools - Options - View. I agree with Sara that there can be good reasons
why seeing at least some on screen can be a real help in understanding what
is going on with the formatting of your document. I like to see the
paragraph marks (pilcrow) and tabs. You can always see what the document
looks like without them by using Print Preview.

Ian