SQL & PL/SQL

DB version:10g release 2

I am currently learning the basics of loop in PL/SQL. I understand normal loops like the one below where a rec (a PL/SQL RECORD) stores all the rows in emp table and emp_dtl.emp_daily_track_nbr is getting updated with sysdate in MMDDYYYY format within a loop.

But can anyone show me a scenario of where a loop inside a loop is used? Is a loop inside a loop efficient?

 --looping through emp rows
 FOR i IN rec.emp.First..rec.emp.COUNT LOOP
        BEGIN

update emp_dtl set 
        emp_daily_track_nbr = to_char(sysdate,'MMDDYYYY')
                where emp_dtl.emp_id = rec.empno(i);
                
 end loop;
 end.