...but really using it to broadly identify different resources or services. In this approach, we take a path like this...
/tools/viewer
...and interpret it as being a request for a certain function of the application. Often, this approach is used because it matches some aspect of how the application is actually organised. Consider this example:
/cgi-bin/script.pl
This kind of thing generally appears in URLs because of the way the application concerned has been deployed - CGI programs live in a particular place and are accessed using a special path "prefix".