do delete/insert instead of update if name changes

hi, all experts there,

I'm studying the example in postgresql-6.5.1/src/pl/plpgsql/test.
I do not understand why we need "do delete/insert instead of update if name
changes". Is it ONLY for the test, or, there is some
reason for doing that in practical use?

I cut/paste part of it below.

Thanks !!!!!

amy
########################################
-- ************************************************************
-- * BEFORE UPDATE on PHone
-- *    - do delete/insert instead of update if name changes
-- ************************************************************
create function tg_phone_bu() returns opaque as '
begin
    if new.slotname != old.slotname then
        delete from PHone where slotname = old.slotname;
        insert into PHone (
                    slotname,
                    comment,
                    slotlink
                ) values (
                    new.slotname,
                    new.comment,
                    new.slotlink
                );
        return null;
    end if;
    return new;
end;
' language 'plpgsql';


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