Simple web sites can do with a simple navigation system. Sites with a navigation structure that is only one or two levels deep can get along with links listed in a sidebar. More complex sites that have a navigational structure that is more than two levels deep would best augment their navigational system with breadcrumbs.
Breadcrumbs are so useful in web sites that have a deep navigation of 4, 5 or more levels. In such web sites, it would be really confusing to the user if breadcrumbs were absent and would look really messy if the navigation was implemented using nested menus.