More specifically, to the user it means they’ll still have the opportunity to back out of the operation; that it won’t take effect immediately upon clicking the menu item. It allows the user to “explore” the command without committing to its execution yet. For that reason, IMO the ellipses in “Attach Files…” and “Add Link…” are appropriate.

What I’m seeing more often in practice is that menu items that should have an ellipsis don’t. They make you wonder what the immediate effect of the command would possibly be.