Trigger time before or after

Question

i did find any real difference between after and before trigger for example
1 create or replace trigger moh after insert on emp
2 begin if to_char(sysdate,'DY') in('SAT') then
3
4 raise_application_error(-20500,'test');
5 end if;
6* end;
SQL> /
Trigger created.
1 create or replace trigger moh before insert on emp
2 begin if to_char(sysdate,'DY') in('SAT') then
3
4 raise_application_error(-20500,'test');
5 end if;
6* end;
SQL> /
Trigger created.
In the two triggers the insert is prevented if the day is saturday if i use before insert or if i use after insert, so what is the difference then????

Answer 1

SQL> create trigger test_emp
  2  before insert on emp
  3  for each row
  4  begin
  5    :new.ename := upper(:new.ename) ;
  6  end ;
  7  /
Trigger created.
SQL> create trigger test_emp_after
  2  after insert on emp
  3  for each row
  4  begin
  5    :new.ename := upper(:new.ename) ;
  6  end ;
  7  /
create trigger test_emp_after
ERROR at line 1:
ORA-04084: cannot change NEW values for this trigger type
SQL>