Shortcut functions are PL/SQL functions usually returning a certain field of an object, given the object_id. All of these functions could be replaced by a simple "join" as part of an SQL query, so these functions are not really necessary. However, using these functions allows to keep SQL queries more compact and readable.
Let's look take a typical example with "im_category_from_id" (the most frequently used shortcut function):
select project_id, project_name, im_category_from_id(project_status_id) as project_status from im_projects ;
This SQL returns a list of all project_id's, together with the name of the project and a human readable project_status such as "Open" or "Closed". Without the shortcut function, the SQL would have to look like this:
select p.project_id, p.project_name, c.category as project_status from im_projects p, im_categories c where p.project_status_id = c.category_id ;
Admittedly, the difference is small in this example. However, many real-live SQLs in ]po[ include multiple categories, so that the shortcut functions provides a real benefit.