SQL & PL/SQL

hi,

i jsut gone through pl/sql oops;

creating inheritance in oops through object type ,by creating supertype or base type and derived type ,is it important to create table of that type and why?
example.

create or replace type person_t as object (ssn number, name vacrhar2(30), address varchar2(100)not final;

create or replace type person_tab_ty as table of perosn_t;

create or replace type student_t under perdon_t(deptno number....);

create or replace type student_tab_t as table of student;

CREATE TABLE test (
regular_field DATE,
person_nested_tab person_tab_typ,
student_nested_tab student_tab_typ)
NESTED TABLE person_nested_tab STORE AS per_tab
NESTED TABLE student_nested_tab STORE AS stu_tab;

why we create table use this nested table .

in type specification why we use not instantiate ? please tell me ?